That is very normal for your betta to do, don't worry.
Are they sitting in front of the tank 'doing nothing' or are they trying to hide? What's the temperature of your tank?
Also, cories like to be in bigger groups, 3 is a little too small for cories to feel comfortable. Is it possible for you to get more cories (like, so you have a group of 5-10 cories?), it doesn't really matter what kind of cory, as long as they need the same things and are about the same size, as cories 'group' with each other, even though they're not the same kind.