Corns don't reach adult size until around three years of age, so three months is still just a baby. It certainly shouldn't be expected to be either long or "buff". At that age you need to be feeding smaller meals, more frequently. There's no reason to have a Corn grow unnaturally fast, apart from lack of patience on the keeper's part. Power feeding will result in an unhealthy Corn with a shortened lifespan.
They'll eat as much as you put in front of them, so don't think they'll stop when they've had enough. Chances are they'll just regurge and then start acting hungry again.
Search this site for The Munson Plan, which is a good set of guidelines to start with.