In nature salvini get about 15 centimeters (6 inches) although they sometimes get larger in aquariums.
3 may work in your tank, with no other fish, except maybe some dithers (no other cichlids), but size is not usually the issue. They can be extremely territorial, and aggressive in small spaces, and even though they are shy now as youngsters, at maturity, or breeding age is when the carnage starts.
If you have a male and two females it may help, or 3 females.
Males usually kill any females that don't immediately want to spawn at maturity.
Be ready with another tank to separate them, as they grow.