Interesting question. I think the first thing to do would be to kill the myth that you "have to play three guys". You play exactly how you want to play. For me, the troupe aspect means that you have a host of potentially playable characters. Every character within Mythic Europe has a story to tell and the players can pick up and play whichever characters they want whenever they want (within reason).
In my Ars Magica lifetime I can't remember any troupe that stuck to a M/C/G play structure. We always had our own magi, a common pool of grogs, and occasionally some companions, which tended to get left in a cupboard and forgotten about. We usually had our favourites within the pool of grogs of course, but not a sense of outright ownership.
So for me, far from being restrictive and prescriptive, troupe play is about freedom. If you want to leave your magus to one side and play his unGifted jealous brother for a few sessions, nobody's going to stop you. You never know, he might gain a little more understanding of his brother's Gift and his jealousy might lessen. And the player has taken the character on a narrative journey, had fun doing it, and is now charged up and ready to play his magus again. On the other hand, if that doesn't appeal, then don't do it.
I think that the best way to think of it is that all the players together are telling a saga about the covenant. There are a number of characters who live in the covenant. Some of the characters are main characters. That is, these are the magi and perhaps the companion characters. Each player controls one or two main characters. There are also a number of minor pool characters that generally don't really belong to any particular player (that is, generally the grogs).
Next, think of the whole saga as being like a TV series (something like Buffy, or Star Trek, or True Blood, or even The Simpsons or whatever it is that the young kids watch on telly today). Each session or story is like an episode of that series. A given episode focuses on only one or two main characters. The others are there, but are only really peripheral to that episode. If your main character is central to a particular session, then that is what you play. If your main character is not central to a particular session, then you mostly play one (or more) of the minor characters that are required for that episode. Maybe taking the odd break to play your main character if she walks into the story to contribute to part of a scene.
Then, as a troupe you just have to manage things so that you are not always having episodes about the same main characters. The idea is that everyone gets a chance to play their main characters. This is much the same way that the director/script writer (or whatever) of a TV series has to arrange things so that the main characters get their fair share of episodes.
One of the best ways to set yourself up to manage this, I think, is for each player to make up a magus character, and the companion character that fits with another player's magus, and a bunch of grogs that generally go with the remaining players' magi. That way, as a troupe you sort of end up with four or five "families" of characters. It is then natural to for each "episode" in your saga to be about one of the "families" of characters, and every player has a role to play and no-player ends up needing to role-play a conversation between two of his own characters. Of course, you might sometimes mix things up, and have an "episode" that involves several "families", or (say) just the magi from each "family" or whatever. But at least with the "families" you have a starting point.
I quite like the analogy of the TV show - perhaps a soap opera is another good example.
An alternative perhaps is comic series like XMen or Justice League - where there are story lines that focus on three of four main characters that go over 2-4 comics, but when they are resolved, different characters get the lime light.
As you suggest, any comic series about a stable of characters is a good analogy too. In fact, any literature/media that is about a stable of characters, rather than an individual character, is a good analogy.
Some of the Icelandic sagas, Irish hero cycles, and the King Arthur cycle, are similar in construction.
There's two kinds of "troupe play" - there's the style which seems to be invoked these days, per ArM5, where the players are encouraged to have multiple characters: a magus, possibly a companion, and hopefully some fleshed out grogs. Episodes within game sessions may switch focus back and forth between different groups. My favourite analogy is a film, with a cast of Stars (magi), Also Featuring (Companions), and minor characters + bit-parts (grogs). A story with multiple scenes may shift focus and show a different part of the cast in each scene (start in the covenant, visit the abbey, meanwhile back in the council chamber, over in the faerie woods,...).
There's also the troupe style which was more heavily emphasised in early ArM (1e and 2e especially), which encouraged the gamers to see themselves as a group of equals, and to share the tasks (story-guiding0 of bringing the world and stories to life. There might be an Alpha SG and some beta SGs, and others be encouraged to run one-off games. This style is not written-out of the ArM genre in 5e, but it is much less emphasised.
The "full troupe" style is much less familiar to outsiders coming in, and in the common case of one enthusiast who has read all the rules + some rules-dippers, and some with an aversion to book study, "full troupe" can be overwhelming and put people right off.
La Roche Caribet played out as the "cast of characters" style, with me as SG, and one other who initially helped by playing NPCs, then joined the troupe as PC, but ran (I think) 2 stories himself. I couldn't get anyone else to take over parts or write/run stories.
It's like when you enter your garage (provided you are a millionaire) and decide: "I know, my Porsche is fastest, but today it might be fun to take the mini, or the pink cadillac."
Fun in gaming is not necessarily dependent on the strength of your character - but choosing the right car / character is a group decision - you can only take the mini if the Porsche driver doesn't get botred and you don't get envious.
Storyguiding in a troupe can involve multiple storyguides (called alpha- and beta storyguides, the one/s with the hairier breasts and the supreme decision power being the alpha/s). This part of storyguiding is interlinked with the concept of the covenant. The covenant is a meta-character jointly run by all players through their characters actions. It is a link between the characters - it holds the players and the characters together and limits individual narcissm. Joint gaming can be fun for everyone.
troupe-style roleplay is for advanced gamers, those that have grown beyond the concept of being the strongest guy on the continent through rules juggling. It is for players that prefer the communal atmosphere of a pen and paper game over the animated monster-bashing of computerized rpgs.
I don't think that this is emphasized less in ArM5. It's how we play it.
One of the roles that each player has is storyguide (along with magus, companion, etc). What might be a bit different is that (I think that) often when ArM5 talks about a "player" this is meant to include the storyguide, rather than being everyone except the storyguide. On the basis that the storyguide is playing the game too.
On a related note, it may be worth stating up front and explicitly that the troupe-style is entirely optional. The game works perfectly well without troupe-style play, and make that clear would help answer the question raised in that post.
I'm sure there would be many things to be said. I wouldn't go back to have only one caracter in any Saga/Campaign! I think this is a pioneering way of doing RPG, really. But at the same time, I haven't found a lot of information on how to play "troupe style", even in the Ars Magica line.
One thing I can add now to this thread, is that we try to avoid situations where one player has to play two characters at the same time. So my companion character is not linked in any way to my mage character. Sometimes we go as far as having them despise each other...
In fact, I would be very interested in any reference to a discussion on troupe play, particularly on the multiple storyguide part of it. Any one knows some more information on this, or are we really alone ?