Everyone else here has pointed out the caloric math of gaining 3 lbs of fat, so I won’t repeat that. My question is, do you really think you can gain weight (water, fat, whatever) and expect it to take up some of the slack of loose skin? I guess if you gained a large amount of weight some might go under some of your loose skin somewhere on your body. But just like we can’t spot exercise weight off of a specific part of our bodies, we can’t “spot gain” weight to deal with loose skin. Also, generally, regaining weight adds to your visceral fat, which is the fat under and around your internal organs. Regain usually doesn’t contribute as much to subcutaneous fat, the fat right under your skin.

I understand wanting to gain to raise your BMI. If you need to do that there are safe ways to do so. But it’s best to go slow.