Growth rates vary between individuals that genetics stuff.Generaly some grow quickly an inch to an inch and a half per month,on the other hand some seem to extremely slow.to optimoze the growth potential of a specimen try the following
1 vary the diet (don't just feed one thing)
2 many small meals thru the day instead of one or two large meals
3 keep water as clean as possible
4 keep water temp in low 80's
At three inches a staple part of its diet should be bloodworms,you can try cutting a quality pellet in half or quarters (single edge razor blades are great for this)
The freezer at your local fish store is a smorgasboard of foods which should include
mysis shrimp
brine shrimp
krill
bloodworms
tubifex worms
squid
beefheart (cut to size for the lil guy)-Anne