I'm a....some sort of queer generally female, in a relationship with a man, and he sometimes does this too.
You need to talk to him about it: after initiating the threesome, he may have realised that it's not for him and that he wasn't comfortable seeing you with someone else, but doesn't want to go back on his word and take back that you're allowed sexual freedom.
You need to evaluate to yourself what kind of relationship your in, and where he stands too: are you poly? Are you both poly? Would you like to make it a freely open relationship for both of you? Or would you like it to remain closed, but be allowed secondary relationships with people that are agreed on with your man? ((This is what I have. I'm not allowed to just go off and have a thing with anyone, and any casual sex is distinctly threesomes only, but I have my ladyshape, and I'd be allowed relationships with other (mainly) women if it fitted with our rules and agreements))
It's hard, even for the most secure people to see your partner with someone else, and if you're both young- you mentioned uni- even more so as you're still feeling your way in life.
It's important to understand you aren't entitled to an open or poly relationship, but it's also important that if you feel sexual freedom is something you cannot live without, you have that opportunity.
I, being basically a lesbian until my current and only male partner, made it clear immediately to him that I'd still want to sleep with women at times, and we built our relationship knowing that. If it's tht important to you, you need to make that clear, and it may mean some tough choices.
So the short of it, is talk to him.
Committed means different things to different people, and Lovebirds post outlines that especially. Her commitment means one person soley. My commitment means to my two loved people equally, within our boundaries. Yours may be different still.
*sings* Talk to him, taaaalk to hiiim, taaaalk to hiiiiim!!!
Also, make sure you weren't making out with hisex/ex fancy bit. That tends to piss guys off :P