To warn you, this might be a comlicated request.

I was wondering if you knew a yaoi where the story first focuses on a pair and a third party who is in love with one of the lovebirds and tries everything to sabotage their relationship. And halfway through, that third party actually succeeds in getting the guy he wants.

Hope I explained it well and thanks in advance. ^^

Takaido Akemi's 35 Do no Renai Netsu kinda fits this I think, although I don't recall much in terms of sabotaging ^^

This is pretty much what happens in Hydra, though the first pair are good friends with the potential to be lovers.

Unbalanced Heat also has a could-be pair that is broken up by a third party.

Thank you both very much. I'll read those manga. 😀