A little salt is fine. Take your time cleaning up your 3 gallon tank, so theres no icky stuff left, and your betta can live in a 1/2 gallon without negative effects. 3 gallons is probably too small to have any other fish other than the betta, though if your lfs stocks ghost or red cherry shrimp, you could try two or three of those.. be careful cause some betta eat the shrimp

Feeding him just once a day would be fine. The tank should be kept around 75 to 80 degrees, and filtration should be gentle - a strong current will stress your betta. Good luck with your betta :D