You didn't "hide" anything. Telling her right off the bat would have been over-sharing. Why the hell would your tell an amazing girl you just started dating about your recent threesomes?

That being said, once you enter into things that aren't cultural norms, you have to accept the consequences as well. You can say all you want about how there's nothing wrong with the casual threesomes, but it speaks volumes that you told your friends about your newfound monogamy and they asked what it meant to your prior open relationship with them.

I'd hazard a guess that there's plenty of reason for your gf's suspicions... It's hardly a normal guess to make. So you weren't wrong for "hiding" it (it happened before her and you told her when she asked), but you arguably were wrong for staying friends with the two of them when there was quite obviously signals being passed back and forth that made your gf uncomfortable. You should have shut that shit down out of respect for your monogamous relationship... She deserves that.