Wanted to comment briefly on your mention of playing with a friend. I just started 5 boxing a month or two back so I would be able to experience all the dungeon content in WOTLK (missed a lot in TBC)... I am in a guild with my brother and some other friends. Ive worked hard to make my setup leaderless and not dependent on any one character so that if my brother is on when I am going to do an instance and he wants to go, I can just drop one of my 5 and get him in the group and run from there. Anyways, if you dont want to do more than 3, not pressuring you, just pointing out that you can downsize your group if you set it up right... anyways, if you dont want to do it I guess thats irrelevant =P

As for the 3 man in a 5 man, I guess I dont really understand your question. Unless you outgear/outlevel the instance I dont think a 3 man of any class set up is probably going to do super well in an instance. (Correct me if im wrong somebody). Im not saying you cant have some success but if your friends arent on then you might be SOL.