How to casually say “on three” in French? Which preposition to use?

Question

Bon, à trois je pars à gauche, et toi à droite.

I wonder if this phrasing sounds good enough? Saying "à trois" rather than"sur trois"? Basically, I wanted to express the idea of "On my mark/count, three, two, one, go".

In German, for instance, you say "Okay, auf drei. Ich links, du rechts", with "auf" corresponding to "sur". In Italian, on the other hand, it would be "allora al mio tre", with "al" being interpreted as both "à" and "sur".

How do French speakers commonly express this idea? Would you use some other phrase?

Answer 1

We never say sur trois, à trois is perfectly idiomatic here.

When counting down (actually up), like with "ready, set, go", we say:

À la une, à la deux, à la trois !

When really counting down, we use the numbers alone:

Trois, deux, un, zéro !