Nah, recipe for disaster. Mollies can be VERY aggressive with Bettas (surprised me, but it happened here, and I would have lost a female Betta to it if I hadn't been watching). Platies are bright-colored, and confuse Bettas into attacking.
Bettas aren't really stupid, but they are aggressive with things that look like them. Okay, they're kinda stupid, but, for a fish, they're pretty smart.