Question

when do you say a las tres versus son las tres

Answer 1

a las tres=at 3 (time wise)
son las tres= it's 3 (time wise)

Answer 2

at three
it's three

Answer 3

you say "a las tres" for something in the future or something that happened in the past
For example: Vamos al cine a las tres
Translation: We're going to the movies at 3

You say "son las tres" when it's actually 3 currently/in the present time
For example: Son las tres ahora
Translation: It's 3 right now