Is everyone receiving exactly equal attention at all times in a three-way relationship? Of course not. That doesn't happen in two-way ones either.
Ah, right. Not that I disapprove that kind of relationship, I just can't imagine how does it work...
I'm a bit curious, but asking about it maybe rude..
Part of it, as I mentioned above, is recognizing that no relationship will be completely even. There's give and take, and honestly it's just adding a little more of that into the mix. Yes, it can be a little tough setting aside the whole "I should be the only one in my partner's eyes" when that's what you've been taught literally your whole life, and giving that much trust to your partners is similarly difficult. But there are definite rewards and some people make it work very nicely.
That's a very general look at it in any case.