Look into clown gobies, two spot gobies, neon gobies, six line wrasse, bicolor blenny, firefish, pygmy angels, percula clowns and many more. They will all stay small but its really hard for me to say you can have 3 fish or 6 fish because it really depends on what fish you decide to get. The inch per gallon thing is an ok guidline but I rarely go by it because some fish are really dirty and others actually help keep the tank clean. It really depends on what you get.
I suggest you look into a pistol shrimp/watchmen goby pair. The pistol shrimp is essentially blind but s the name sugguests has a claw that can pack a punch. The watchmen goby on the other hand has great site but is pretty defenseless. They form a really cool symbiotic pair and never leave each others side.
Depending on which shrimp/goby you go with you could probably do two/three more fish.