Bikes for Christmas

Sarah • Mother of a 6yr old boy and a 3yr old girl and a baby girl on the way
So I'm considering getting my 3 kids bikes for Christmas. Not sure what size to get them. My boy will be 3 I'm thinking a 12" my daughters are 5 and 7 so I'm thinking possibly a 14" for the 5yr old sounds she is only a little bigger than the boy and possibly a 20" for my 7yr old since she is tall. Any suggestions?