if you are gonna go with corys in the 20 long get alot more the 3. they are schooling fish, and 3 is the ABSOLUTE minimum. Ive seen alot of people with 6 in a 10 gallon, so i would start from there and move up. the more of them they have, the happier they are, and they will show their schooling behavior which is cool to watch. 1 side note on introducing everyone. take your betta out, rearrange the tank a bit and add the other fish first, and give them some time to get comfortable. If you were to just add the cory cats he is going to be aggressive because he thinks the tank is his territory and that he has to defend it. if you rearrange it he will think it is all new and thus not his territory. it will cut down on his aggression.
My Girlfriend and I have these Bettas
Wannahockaloogie-Male VT-Blue w/ purple tips.
Mist-Male CT-Dark Green/Blue.
Kitty-Female Delta-Purple Cambodian
Nala-Female VT-Orange Cambodian
Hippie-Male CT-Red, White, and Blue Dalmation
Puddles-Male CT-White w/aqua fins and light red tips