Hello Jozel, I too am really sorry that this is happening supposedly behind your back but the truth wins whether he wants to accept it or make excuses but there is no denying the fact that he has been doing this for 3 months and 3 times a week.
If he says he's working overtime then he has to show you his payslip, which I doubt will happen and if for some reason he does show you, then I wonder whether it's pretending to his boss that he has been working, when in fact he's having an affair and if the boss finds out then his job may be in jeopardy.
He's not only doing this to you but also his two teenage kids who would probably know by the way he acts that something is not right.
Eventually, he has to be confronted and a suggestion is before RA because if it's not then he will only make up an excuse why this is happening.
Best wishes.
Geoff.