Thanks for your supportive postings. The holiday i mentioned is all booked now (although it took some explaining why we wanted a room with an extra single bed that could be pushed together with the king/double to make one very big bed!!! lol)
The "third guy" (or my 'other bf' as i should probably call him) is an officer in the Armed Forces. He's had mono-relationships in the past which havent worked out for him because the times he's been away (6 month postings) have proved too much for the bf's to be on their own. As i see it, this kind of setup is ideal for him as he has a 'home' life which can function when he's away too. I think from the small comments he's made that this is his perspective on it too.
This also means that my primary partner and I, as much as we miss the third guy when he's not around, get alone time together too. So nothings really encroaching on any "private" time between us either.
Incidentally - a few days ago my sister (who i havent shared this with) asked me outright if i was having an affair with the third guy behind my partners back becuase she'd seen the way we were around each other. LOL. if only she knew!!!!
Thanks again for your comments, its really nice to know there are people who think relationships like this CAN work.