How do you find one of the three legendary dogs in fire red?

By the way, they are the legendary cats, not dogs. The Pokemon Company revealed that Raikou is a tiger, Entei is a lion, and Suicune is a cheetah.

Use sweet scent in the grass near the tower and they should appear but be cutiOS they only appear once in the entire game

Acctually, that's not true. They also appear @ random in the wild, cuz as i was walking, a wild raikou came up to me.