Please help my cat has gave birth to 3 kittens 2 have dyed already she isn't showing an interst in the 3rd why

Do the kittens have fleas?
Wet around their thighs, stomach and tail if the water turns red they have fleas and the fleas will kill them.
BUT you can not use flea shampoo on them.
Another problem may be worms.
Also if the mother is not feeding that can cause them to die. You can buy kitty milk and feed it to them

You really need to take it to a vet though

She may be too young, the kittens may be genetically deformed in some way and she senses that it's pointless.

It's time for you to take control.

You need to get the mother cat and remaining kitten to a vet asap.

The kittens could be born too early, or it could be that the mother pretty much has abandoned her yound.

You should take the kitten to the vet to be treated, and most likely by kitten milk and feed the cat yourself. That way she gets better.