You should get tested cause condoms protect you from pregnancy. But they don’t completely eliminate the spread of skin to skin infections. It is wise to discuss your sti/std status with a partner before you have sex with them. So nothing just pops up out of the blue one day. 3 weeks is good. So is 6. So is getting tested regularly (maybe every 3 months) if you are really active. Oral, you can add a barrier, but if not there is a chance to spread stuff. But the best preventative measure is asking before any fluids are exchanged.