I'm glad things have been going well for you two, I wondered now and again how that turned out.
Don't you think that you've kind of become him, pressuring him to spend time with somebody (threesome) when he isn't that interested in it? It's good he figured out that he didn't want to spend that much time with her before the trip got closer, and yes, he should be the one who tells her. She should also get to know (if she doesn't) that he just considers here a body to fuck though you think it'd be obvious to her. That's the mature thing instead of "keeping her at bay" when she wants more, but apparently she find it satisfying enough to stay around.
So after he delivers his own bad news - what is the problem? Just date her on your own. Only think I can think would be if you were relying on him to foot the bill for travel so you could all get together, but other than that, it might be awkward but life is awkward. I don't see what's wrong with you getting closer to her. I don't see that he's trying to get farther away. He has what he wants with her now before you "butted in"
Casual sex with her a few times a year. No reason he can't still have that if she still is interested after this speed bump, no matter what you and her end up having.