In Episode 3, Why did Mami didn't escaped অথবা did anything when পুডিংবিশেষ 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 পুডিংবিশেষ was stronger than Mami, which means they had at least one পূর্ববর্তি encounter that ended with Mami either beaten অথবা dead.