Hey @Kloaker! Welcome to the forums!
So this is standard array stuff, no worries!
First, check if TraitNames LENGTH < 3. If that is true(branch):
GET (a copy) of your list of possibilities array, get random, and Add Unique, (not just Add) to your Character’s list of TraitNames. This prevents duplicates. Then at the end of that, loop the execution line around to before the LENGTH < 3 check. Once it triggers “False” meaning it has >3 traits, it will break the loop and you are done, the False is your exit. 
Hope that helps!