I’m trying to add 3 20 amp exterior outlets , and was wondering if I could add a 20 amp breaker to my panel, use 12 gauge THHN wire, in pvc conduit. The exterior outlets are in weatherproof boxes. Thanks for any help.
Assuming you have space in your panel for another breaker, sure.
You'll either need a GFCI breaker, or GFCI protection some other way.
I'd suggest either an internal outlet on the same circuit before the outside outlets with a GFCI in it, or a dead-front GFCI (test and reset buttons, but no outlet) located inside. Putting GFCIs out in the weather, even in "weatherproof" boxes, tends to shorten their lives (even if they claim to be "weather resistant" themselves as well.)
Both of those options are usually less expensive than a GFCI breaker.