What, no words of wisdom to precede mine? I have a blank canvas!
Personally I find any choice other than learning to trust depressing. Without that trust paranoia will rule the day.
But the top three choices all have some merit to them.
"Terrible choice in women": Three cheats out of three means Our Guy is looking for something in a woman or is relating in a certain way that he shouldn't. Something toxic. A little self awareness as to what it is will go a long way toward rocognizing what that bad stuff is and avoiding that path
Maybe he subconsciously thinks a "good girl" is boring.
Maybe he's drawn to the drama of the confrontation with the cheatin' girl.
Dunno. Moving on.
Trust, once lost, is really hard to get back. Being vulnerable is sometimes difficult and uncomfortable when you've been burned a lot. Consciously cultivate it or it will never return.
Let's not be stupid. "Trust" doesn't mean "blind trust". If she's ceating, she'll leave clues even if she's trying to hide it. Our Guy ought to be able to tell what they are. "Trust" means not blowing a single incident out of proportion.
Look for patterns of behavoir. Our guy ought to have previous examples to work from. If she's busy when you want to go out once, that's nothing. Life happens. when it starts to become a pattern on a certain day of the week or month only then consider the possibility.
You see, "stupid" includes Our guy allowing his fear to run away with him, making himself paranoid or possessive. By giving his fears free run, Our Guy could end up turning a nice girl into the very thing he fears she might be.
Our guy should talk to his girl about this. Make sure she knows he's been burned and is prone to jumping at shadows. See what she has to say to reassure.
Finally: Our Guy? Trust yourself. You are allowed to have a happy, stable relationship. Relax. Enjoy your girl while she's there. Make some good memories you both can take away, no matter what. If she's cheating, the signs will be there. Trust yourself that you'll see them.