I think both sides of it are quite natural; they had the time of their lives and they want to talk about it, yet for you it's annoying because a) it gets very repetitive and b) you didn't get to go.
My suggestion probably sounds quite obvious. The next time you're out and it happens either wait for a slight lull in the coversation and attempt to change it, or if you're feeling brave, come right out with and tell them that you'd rather they didn't talk about it constantly.
If you were friends with them before the trip then I imagine you do still have a fair bit in common with them. That being the case, think back to the things you used to talk and laugh about and bring these things into the conversations.
If they really insist on talking about nothing other than their trip abroad then your options are to register an interest in going with them next time, try to get involved in their stories and experiences, or make sure there's at least one other person going along who you can engage in conversation of your own.
The old saying "two's company, three's a crowd" is absolutely right. It doesn't need to be a nasty thing, it's just that two people in a group of three will generally have something more in common than the third person and it tends to lead to a bias of conversation. The common thread for your friends is clear.
I don't think for a second that there's any intention to make you feel like you're not part of their conversation, but it is a little unthoughtful of them for sure.
Christmas to a child is the first terrible proof that to travel hopefully is better than to arrive - The fabulous Mr Stephen Fry.