I hate to be the one to tell you but if you have a betta and two goldfish then you should have at least a 30g tank, Yours doesn't look that big
Goldfish grow big and live long, they should have 20g for the first fish and 10g per additional fish. They are also messy fish and need very good filtration to cope with the waste produced.
Do you have any filtration for the tank and just how big is it?
Goldfish and Bettas is a no no, but if you have VERY good filtration that somehow doesnt disturb the water too much, they may live along happily. That is until the goldfish get big
what r u feeding them.cause they take 2 different kinds of foods.dont they.instead of goldfish.add some snails,ghost shrimp,an african dwarf frog.they all will go nice with the betta fish