Good call on the 3 gallon! I kept a Betta in a 1 gallon corner tank for 5 years! He seemed very happy and very vibrant colors. I had an air stone and no heater and no lights. He was stationed on my headboard of my bed on the shelf. I even moved and threw the guy in the back of my car with half water to move from Alabama to Indiana. Made the trip and lived 4 years with me in Indiana. I missed the guy! I did change the water out every single week and I would clean the rocks with hot water and the plants. So I am guessing my tank never cycled. I am not saying a 1 gallon was good for him, but he seemed very happy and grew up with very big gorgeous red fins. Looking back I think that I got lucky being I didn't really know anything about keeping fish and never cared to learn. I just kept cleaning the whole tank once a week, and I did feed my fish bloodworms and Betta Pellets as well as a flake food. So At least I gave him a good diet! My last attempt at a Betta resulted in failure. I had a 2.5 gallon tank, with a small filter and heater and lighting. He decided to jump from my tank. :-( With your 3 gallon, you Betta should be grow up to be nice and big! Feed him live bloodworms here and there. They go nuts for live bloodworms! I think pet smart has some live in containers. A good flake is good, but you should feed your Betta mixed variety of foods, not just flakes. You can even get frozen mysis shrimp and feed that to your guy as well! They come in a cube so you will need to cut it and refreeze the amount you don't need. One package of frozen mysis should last you for months being you won't need to feed him the whole cube! Thaw before feeding.
I use little Dixi cups to thaw the frozen food! If the small amount you would feed to one Betta, I am sure you can use something smaller! Like the little cups you get for ketchup at McDonald's! I just dip my cup into the tank to get a little water and once the food is thawed, I throw it in.
http://nippyfish.net/bettas-101/feeding-bettas/