I vaguely a manga i once read but cannot the name b1d33

It was about this guy (A) that liked a classmate (B) of his.
There is also this other guy (C) in his class. Not sure whether they are friends
I'm not sure whether the present is when he is a adult or during his school days but he has the ability he later finds out "to change events in the past" or, depending how you look at it, "see how certain actions turn out in the future"

Among other things in one timeline, C has become a pornstar (if i correctly) and B has become C's girlfriend

Another thing that happens is that A is having sex with a classmate while C is having sex with B. This happened near a swimming pool if i correctly

A starts manipulating events so that B becomes his girlfriend. Not sure whether he was eventually successful

It's not very recent so at least a couple years old... could be as old as 15 years ago...

Well i've found it

Takuwan - Turning Point