NO. If he is flaring at them, he may be getting ready to take a hunk out of one of your platies. Remove the betta immediately. While some betta will do fine with other species, for many, this behavior is the precursor to dead or very injured fish. Do not put it off any longer. Move your betta right now.
I have a crowntail who loved his guppy and platy friends. They did very well together. I tried a different betta and he displayed the same behavior yours is showing and chased down one of the guppies. We moved the betta before he could reach a guppy, but it was very close. My crowntail is pretty uncommon though. Move your fish.