I had a similar situation a while back, actually.
I think that it's good that you are willing to be friends with them
again, despite all that had happened.
To be honest I don't think that A and B should be mad just because you
and C hung out together. I mean, you can't do everything together.
Also, I think that in the future, instead of excluding them straight
away, try to talk to them about it first. This is kind of like
fighting fire with fire and so far it isn't the best way to deal with
this problem, right?
It's hard to fix things when not everybody is willing to listen. What
I think you should do is to talk to C about it first and make you both
genuinely want to make up with A and B. Then you should try to arrange
some time to talk to A and B. Do your best and try to get them to
listen to you, but make sure to let them have their time to talk as
Both 'sides' of the situation have something in common, so maybe you
can try to talk about that if ever you guys get to talk. You both
thought that you were excluding each other, when that wasn't really
the case. Try to clear out that misunderstanding and then you can work
on things from there.
If that doesn't work and if you can't find the time to apologize and
make up with each other in one go, maybe you can do things like, give
them time or try to let them know you're sorry in small ways. Take one
step at a time. If you all really want to get together as good friends
again then a misunderstanding or a situation shouldn't be enough to
separate you from each other, because friends shouldn't let something
like that get in the way of each other.
I'm just giving you some suggestions, I can't guarantee that what I
say will help you, although I hope it will. ^^ Even if it just gives
you a little bit of inspiration, then I'll be glad. I hope you all
will get along soon, and good luck!