I'm going to repost what I put in another comment:
Let's review: wife wants a threesom and uses some manipulative techniques to pursue it, husband reluctantly agrees to threesome, baby starts crying during threesome so husband goes to tend baby, husband falls asleep tending to baby, by the time husband wakes up all the sex is over and wife and friend are asleep, husband gets mad.
Does OP have justification to be pissed off? Hell yes. But goddam, OP did agree to the threesome, even if hesitantly and even if manipulated. He walked out on the porch where wife and friend were making out and gave the nod to the friend. To take your analogy of a friend driving drunk and getting in a wreck - if the wife was the drunk driver, OP was a willing drunk passenger, up until he had to get out of the car to take care of necessary business, and the shitty part is that the wife drove off without him. It's not a very fitting analogy but I'm working with what you gave me, heh.
Let's be real here and consider the fact that if it weren't for the baby waking up OP might have awakened the next morning possibly pretty stoked about the threesome that happened the night before.
OP's wife did bad shit. No doubt. She should not get a free pass. She wanted to fuck, and not her husband. She wanted to feel desired by "other men". Honestly, I can understand that. As a married man I still like to think that I am attractive to women. Hell, if my wife gave me a pass, I'd love to roll in the hay with other women. And I can understand how OP's wife might feel the same way and might feel the need to feel attractive even moreso after a pregnancy. I think people are blowing this situation out of proportion - and it is something of a bad situation so the last thing is needs is to be blown out of proportion.
The fact is that OP was going along with things. OP's wife fucked up - she fucked up by not making sure OP was included in the threesome the way he was supposed to be. She's made a genuine apology and shown genuine remorse. I see this as a something that can be recovered from.
I think everyone needs to take a deep breath and remember all the circumstances of this fucked up situation.