How should I say "3 2 1 go!" in German? For example, in a race, start running after the countdown ends.
As an equivalent to "go", there's the adverb los. It basically is a request to start something, to hurry up, to get going etc.
Los, beeil Dich!
Come one, hurry up!
In a race, "los" can be used in multiple ways, for example
Drei, zwei, eins, los!
Three, two, one, go!
or in the opposite direction
Eins, zwei, drei, los!
The latter is similar to the English "on the count of three". With this variation, the "three" is often used as the "go" signal, omitting an explicit "go": "Eins, zwei, drei!"
You can also use a more "race specific" lingo, that you may encounter in official races:
Auf die Plätze. Fertig. Los!
On your marks. Ready. Go!
Achtung. Fertig. Los!
Attention. Ready. Go!