Y is it that kittens open eyes after 3 days?

Actually, all the kittens I've ever had opened them at 7-10 days. The fact is that kittens are born fairly immature. Not as immature as rodents, so there's more investment in the mother's womb time to grown them, but not as much as a foal or a calf. The mother won't abandon them quite as easily as a mother mouse would her young because of that investment, but will go into estress soon if she lost her kittens. A kitten is unable to do anything to protect itself much before 4 weeks anyway and at that age, only to run into hopefully a small enough hole to be safe. Without a parent's protection any young will die, but in some ways, the size of the litter makes some expendable so long as one reachs adulthood to reproduce. Consider, that it is only because of our expanded brain case that human young are born as early as they are - all other primates' young are much more capable than baby humans. So you have baby mice - with out hair, eyes not fully developed, cats and dogs, that do have hair, the eyes are developed, but not open, human babies, fully developed, but very helpless, ape young that can cling to their mothers at a few days and horses/cows/etc whos young are on their feet in a few hours and can run in a few days. Only human young have a whole society that should be protecting them to be born so early, the other early births are in part to be expendable if they need to be.