oh did you blackmail her in town then accept bj at tower.
sounds like you took the same route i did. i am not really good at finding triggers. still have idea where isabelle's first trigger is. anyway im on good path and i have not got the threesome either. Just retried it with mida there and just rent no blackmail no good. so you might be right about Mida being as the problem. if you've started the blackmail route got temple and both library scenes plus rent scene.