You have a great advice and i hope your betta will get better.
Just want to say definitely do more water changes for your other 2 bettas in .5 gall. Especially if you are feeding them with flakes. Flakes are so massy and a lot of them probably sink to the bottom while they are eating them. Always remove uneaten food. And I would still try to offer them pellets. A lot of time bettas refusing to eat pellets at the beginning but if you keep offering them, they might eat them eventually. The brands that i think are really good are Aqueon Betta Food; New Life SPECTRUM Betta Formula.
Also make sure when you change the water -new water is approx the same temp +-2-4* as their water.