Sometimes the filter flow is too much in a small tank. You may want to consider turning it off or reducing the flow for a while. Also for a tank that size, even with a filter, you should be doing a minimum of 2 water changes a week, if you turn the filter off, you'll need to add an extra water change. Check out this sticky for help: http://www.bettafish.com/showthread.php?t=115758
it will tell you how often and how much to change based on your tank size, filter or no filter, live plants or none, etc.
And by lots of hiding places, I mean lots. Most would consider 3 not very many. Betta fish need a lot of cover and hidey holes. Both of my tanks have more than 10 hiding places, and for my male, that's not enough - I'm going to have to get him more. He LOVES to hide and his favorite hiding place is behind his filter, but he also hides behind his heater and on or under all the large leaves from his Lilly plant (there's about 9 of them). He used to hide in his pineapple home, but I had to remove it because the paint was chipping off, so I need to get him a new hiding home to replace it. He also has a fake plant with large leaves that he hides in.