I have looked and looked and I can not find a single official rule in fifth edition describing what happens when one combatant is attacked y two other attackers or what happens if a group attacks a single person. Do any official rules exist? Am I completely blind?
What are you looking for exactly?
In anticipation of it being how one defends from two attackers, I think it is subtle, but attacker and defender roll and compare totals. Therefore, someone can defend against multiple attackers. Negatives to defense roll? I don't think they are necessary, combat is dangerous enough.
You haven't missed anything obvious, because it isn't clearly spelled out. However, combat in ArM5 is either between two individuals or two groups. In this case you have two groups (rules start on ArM5, page 172): one with 1 member and the other with 2 members. Each group gets one attack against the other in their turn. If the larger group inflicts damage, then the singleton takes two wounds (he is his own vanguard).
The only bonuses that might result is if the larger group counts as a Trained Group, in which case the bonus is equal to the lowest of either the non-vanguard's Weapon Ability or three times the designated leader's Leadership Ability. This bonus can apply to the Attack or Defense Total. The singleton gets no Trained Group bonus because he has no other members in his group to generate it.
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Except of course that that only works if the 2 people attacking are within 5 points of combat bonus of each other. If they aren't, you can't use group combat and then the rules give you nothing to go on. (Yup, I've run into this problem a few times.)
Either way, the 2 attack twice in a round and the 1 only attacks once, so he's in trouble. Technically, the group rules have one "attack roll", but thematically it is two attacks - that's why he is wounded twice.
There isn't any other downside for facing multiple attackers. As Jonathatn.Link said, Ars combat is dangerous enough that this shouldn't really be a problem. If you insist on this bit of "realism", however, I'd suggest introducing as a house rule a -1 to -3 penalty to the character's Defense per additional attacker. For groups, apply the bonus per multiplication of the group - so with a -3 penalty rule, a group of 2 would be at a +3 advantage against a single opponent, and a -3 disadvantage against a 4-person group.
This is for melee combat; you then want to think about missiles. The simple option is that missiles don't count. If you insist that, too, isn't realistic, you can pile on archers as "opponents", or apply a flat (-3?) penalty for being on guard against arrows while fighting.
[Edit - slight correction; my first sentence got messed up.]
The group rules are their to make combat easier to run not being able to use them mostly just makes combat harder to run. If a collection of combatants don't qualify as a group then I just roll them separately. Everyone get's one attack and one defense roll so if an individual or a qualified group gets attacked more then once I just compare each attack to the single defense. Their may be some benefit or handicap to treating separate combatants this way even if the qualify as an untrained group but frankly I don't really want to crunch those numbers.
RAW you get a separate regular defense roll against every attack made against you. There is no more penalty for multiple attackers as there used to be in previous editions.
... but then 5th edition combat is simplified to the point of blandness. There's lots of stuff in there that doesn't make sense. Lords of Men adresses some of those issues, but in the end it's best if you houserule the combat to your tastes. Just make sure everyone's onboard with your changes.