I avoid sex with girls who had higher than 3 sex partners to reduce chances of stds, is that a crazy way of filtration? if not what's the right number

That is a good idea.: Statistically speaking, the fewer the partners, the lesser the chance of having STD's. Of course, it goes both ways. The boy cannot have any more partners than the girl. That's the rule. No excuses. It's called gender equality.

It doesn't matter: People lie, all the time. What you makes you think you can believe when someone tells you they have only slept with 3 people. Are you completely honest with them as well? Anyway, use protection - condoms- every time all the time and that is the best way to decrease your chances of getting anything. Anything else and you are only playing games...