Its only licensed for chickens over 6 weeks of age, and i wouldn't advise its use in 3 week old chicks- it may have side effects.
I'd actually recommend a much better wormer called 'Flubendazole' (Flubenvet) as this is safe, and can also be used in laying hens, whereas Wazine cannot.
I didn't realize that you couldn't get Flubenvet in the US, what a shame, its very good- i use it with my own small flock.
The main reason with waiting with the Wazine till the chicks are older is that it can cause some neurological side effects in very young animals- so its not just that it would be off label- i wouldn't want you to make any of the chicks ill inadvertently.
All the best Andrea