Sticking witht he general rule of 1 inch of fish per 5 gal, I would say that would be pushing it. You would have to be very dedicated to water changes and upkeep for a 12 gal anyway.
You could probably do the 3 fish if they are bought small and you have at least 15 lbs of LR with 3 to 4 inches of LS, to make up for the bio-load..