Does 3 y/o and under have to pay $10.50 + $5 rental as well?

Yes, If you want to skate you pay, no skate no pay. I think children under 12 years different rate than adult rate.

Rental fee for sure but I'm not sure about the entrance fee

Best bet check their website

I believe so, if they intend on going on the ice. My daughter is 4 and has to. Not sure if there's a cutoff age