I have 3 identical pure black kittens and 2 identical pure white kittens. How to tell them apart.?

Same happened to me! Try to wait a bit and you'll notice they each have little differences in paw and facial markings

Ahh, there's no rush or hurry here...they will be with their mother for at least 10-12 weeks anyway but day by day, if feeding well, will grow considerably and change to show very slight features starting to appear and also personality traits which can be another way to tell them apart more easily. Just watch them, let the mother feed & look after them. No need to tell them apart this early on. Give it time.

Whether male or female can be quite difficult and normally takes a good expert or vet/professional. Again, no hurry. They will require their two kitten vaccinations so at the first one, you can ask the vets to help you.

buy some different color string like you get on balloons and when their EYES OPEN loosly tie a string around each of their necks. (not to long to get their paws hung up in and not to short to choke them.) In a week change the string to a longer because they will grow, if you can stick your middle finger under the string then you know you have the right length.

Buy the kitties little collars, each a different color. I personally don't like putting collars on animals, but maybe until you figure out the subtle differences each cat may have, you can stick a collar on 'em. :)

awwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwwww.

I would get them different colour collars. But as their owner you might find little quirks and things they do that are different to id which is which. Get them mirco chipped too

you can loosly tie different color ribbons around their necks or feet. You can also get different color collars to tell them apart.