I'm taking care of 3 baby squirrels, 1 is twitching a lot—is it having seizures? If so, why is she?

You should just take them to your vet or to some place that takes care of wild animals until they're old enough to be released back into the wild. You said it yourself that you have no clue what to do, so the best thing for the animals is to take them to someone who has the knowledge and experience they need. Good luck.