Yes that's true, it's a nice gesture of OP to wait that time but he doesn't have too if he'll hurt too much. It's a considerate course of action but it's not always that simple, lying for 3 weeks e.t.c. although hopefully she'd be so distracted by exam prep that he can mostly keep his distance without arousing suspicion.

As for your case, given she'd cheated repeatedly, not taking into account your feelings so it's understandable for you to be a little selfish for once and try and salvage as much emotional wellbeing as possible. 

OP case is different because she's done nothing so bad but it is 3 weeks away, not the same as the day before or week before examples that people talking about.
If it was a couple days before the exam then OP really should wait that long so as not to potentially ruin her life (as she hadn't done anything horrible) but 3 weeks is a long to pretend so it is reasonable if he doesn't want to wait.