I had some issues triggering this as well.
Particular flags are:
In the bathroom
Not a female (using the male story)
Done Sasha_breast_complex_2 (pool scene)
Sasha is valid (I haven't tracked down how this is used)
min_love is 150 (over 75 love points)
min_sex is 1 (had sex once, I believe this requires a date)
cheated is False (haven't cheated on her)
Easiest way to check things from the console:
Code:
EVENTS["sasha_breast_complex_3"].test(True)
I know I was able to activate the event by going into the bathroom when no one was in there, at 10 on a Tuesday with both harems and Sasha pregnant.