This is driving me crazy and I can't seem to locate the author or title but I read this in an anthology and would like to read it again.
Three young children, 2 boys and a girl, become super-intelligent psychics that kill off the rest of the world because they are a threat. The girl decides they should mate in order to preserve their powers only to discover the baby she is carrying is more powerful than any of them (it speaks to her in utero) and will probably kill them off once it is born.
Most of the story, IIRC, centers on the 3 children and how 1 of them doesn't like that the little girl killed all the adults (and everyone else) because he misses his mum and dad.