I was given an underfed corn snake that is 3 years old very small?

A snake puts on weight best when fed a couple smaller prey items than one big one. So if the snake's eating hoppers you might want to switch it to two fuzzy mice. I'd feed every 5-7 days. There is a difference between a snake that's underweight and one that's just small. If it seems to be a healthy weight for its size then I would just keep to a normal feeding regimen of once a week and the snake will grow in time. If it seems underweight you might want to up the amount as I suggested above. As long as you're not constantly feeding the snake there's no danger. A good rule is to wait for the snake to defecate before you offer food again. That way you can be sure that it's digestive system is finished with a previous meal before you give it another.

Feed one prey item every 7-10 days. There is little benefit to feeding excessive amounts except putting fat on her which is not good for her. You want to slowly work her system back up and for snakes, they aren't animals that normally eat weekly. Her feeding schedule was probably closer to a wild counterpart than you think. As long as her spine is not sticking up (and do not mistake the muscles of the spine with the bones) and her ribs showing, she's probably not starving as much as you think. I feed my adults every 2-3 weeks and sometimes close to 4 and they are all in excellent condition. The only exception to this feeding schedule is hatchlings and gravid/post laying females. She'll be fine if you take it easy and don't worry too much.

i did the same thing with my ball python...my breeder that i know said the fastest way to put weight onto a snake (and from the guys from snakebytes tv) all said feed twice a week. for exp, my bp should have been on small rats, i feed her a mouse on tues and fri. she did put on weight quick, but not to quick...GOOD LUCK!!!!