If you already have the 3 cories and the betta in there I would get some shrimp.. or maybe a snail..I think that may be fully stocked though (I could be wrong).
I was talked into getting 4 cories for my 10 gallon..I feel like I am way overstocked now...
...I plan on moving up to a 15-20 gallon as soon as petco has there 1dollar per gallon sale again..
Also, depending on what type of gravel you have..you might want to consider switching to sand...cory cats looove sand! It's real cute to watch them dig their little faces into the sand!