It looks to me as if you've just got a surprise marble on your hands. I had a boy that did this to me a while back....even dark coloration, no indication of any sort of marbling, and then BAM he started getting a couple light patches and changing just like you're guy.
If his behavior is normal and hes eating just fine, I'd say thats all it is. ;)
.....But you have two females and one male betta in a tank together? Without a divider?
That....is really a very bad idea. I'd highly HIGHLY suggest separating them ASAP. This strain of Betta Spendens that you find in pet stores are a very aggressive and territorial species, selectively bred for years to viciously fight. By no means should you EVER house males and females together expect in very special circumstances(like if you were a very experienced breeder with a very large tank and sibling fish you have bred yourself).
They may appear fine now, and even fine for several months, but everything can change at the drop of a hat and I can promise that it will end in a bloodbath and I would hate for you and your lovely fish to have to go though that. :/