In Episode 3, Why did Mami didn't escaped o did anything when charlotte was about to eat her head?

The attack was unexpected and likely simply too fast for her to dodge effectively. I'd say both of these contributed, but we don't know the specific reason. We do, however, know that Homura was aware that charlotte was stronger than Mami, which means they had at least one anterior encounter that ended with Mami either beaten o dead.