People are right that it can be a little difficult to introduce a third, but it's worked successfully for me and my boys. First, years ago I met Thomas and we spend a long time together, tried a third and it didn't work, and then Chris came into the picture, and things were good for awhile--but my relationship with Thomas was crap, and eventually we broke up. I had my eye on a boy I'd started to know, and I brought him home to live with us, and now the three of us are quite happy.
In our case, part of what made it work is that I'm the dominant of the house (they call me 'King Lion' ;3), and the choice was mine to make nearly exclusively. So, that made some things easier, and by choosing carefully and taking time to make sure Chris could meet him, and spend time with him, and then the two of us could talk alone in between, we found it to be comfortable.
I think that, if there are needs to be met that aren't, then being the guy 'cruising' for the newer partner, on your own, can be a big part of finding the right man. The foundation of trust there rests on your partner and you both knowing that, if somebody comes around that you like but they don't fit in the group, that you'll walk away.
In my experience, I've found that leaving the door open, as it were, is the most important thing in keeping a relationship fresh, honest and up-front. When somebody finds leaving unimaginable, they may do any number of things to keep things 'normal', to keep things stable--but when leaving is just as easy as can be, then telling the truth and letting worries or problems come to the surface right away becomes the only sensible thing to do, and it makes you a stronger couple for it.
I wouldn't give up just yet . . . but my focal point is, whether you do or don't, the desire to have a third can stand just on its own and that's ok.