Have you thought of live food? peas are a really good food to give your betta, just make sure you take the skin off the pea and cut it if your betta is smaller sized. black worms are really nice food for bettas and they're pretty cheap (where i'm at they're like 80 cents per scoop, and you'd need like one scoop for three bettas and that will last a bit)

betta flakes are a joke, so i wouldn't buy the flakes but i do feed pellets now and then as well as freeze dried blood worms and, since i live in Minnesota, mosquito larvae.

put a couple decorations in the tank. i have a few plastic plants, three really small pots (i found them at a local fish store in the betta section) as well as three Buddhas. I also have a bubbler in the far left corner and my betta loves to float in them. he really enjoys all the hiding places and he's really active because of it. My plakat is only in a 5, but he really enjoys it. i can't wait to get the rest of my stuff so i can set him in the ten i got sitting in my dad's basement.

Adding aquarium salt is really beneficial as well as it keeps bacteria and sicknesses away for the most part and provides essential electrolytes for fish which will enhance coloring and vitality.

I also use melafix at the fist signs of anything natural and it seems to always help.