How many regionals should I apply to?
 

Should I apply to every regional or just my top 3 via AirlineApps?

Depends. If you goal is the soonest class date possible, apply to any you would be willing to work at and take the offer with the first class date.

But there are probably other factors to consider, given that the "typical" will have multiple offers.

So I would first apply to your top 2-3 choices and see what you get. If you have a hard preference for one particular regional, and it has no backlog that you know of maybe then apply only there and see what you get. If it's a class date in two weeks, done. If it's a pool or a class date next year, then perhaps apply to a few others and see what you can get sooner.