First off, it's great that you've taken on this little guy. It's awful the way these large snakes end up getting dumped
I woudn't recommend force-feeding the little guy yet. That is a last resort to bring back a snake that isn't going to make it because he's so long gone without food. It is extremely stressful on an animal.
You should always try and feed either prekilled or frozen/thawed prey items, since live prey can inflict harmful, and in some cases, fatal wounds on an animal. At 3' he should be eating rats now. Mice are too small for a snake that size. A snake can eat the equivalent of the largest part of its own girth (well, they can eat larger but it isn't recommended can can result in other problems such as regurgitation, etc).
Before you can expect your snake to feed he has to be in the right environment. I would give him a hot spot on one side of his tank @ 90 degrees, and on the other end of the enclosure it should be about 78/80 degrees. Make sure he has many tight fitting hiding spots so he feels secure as well.
I would feed him once every 7 days at this age. First try dangling a prekilled rat in front of him (at night is your best chance for him to feed since they are nocturnal) with a pair of tongs. If he doesnt take that, then jsut leave it in with him overnight and cover his enclosure so its dark. If that fails put him in a small dark container with the prekilled item overnight. When he does start eating be sure not to handle him (except to put him back in his cage if he ate in a box) for 48 hours so he can digest. If you handle before this you run the risk of regurgitation. If that doesnt work, try putting him in a small box with a brained prey item. Braining is when you cut into the skull of the prey item and smear a little of the brain matter around on the prey items head and nose area. Leave him overnight.
Try to avoid handling your snake until he has begun feeding regularily for you, since they can get quite stressed from handling. It's best to keep him in a quiet, low-trafficked area until he is feeding.
Good lcuk with your new little guy, it's great that you're taking him on! :thumbsup: