well..
the idea that
"it takes 3 weeks for gear to kick in"
is based on the fact it takes about 3 weeks of injecting to reach the apex concentration of hormone circulating in your body.
testosterone, regardless of ester, starts working from day one(starts releasing test into your body).
but on day one, the additional testosterone will not yet be enough to make a dramatic impact.
an ester like enanthate starts to releasese day one, peaks about day 3-4, and subsides slowly down to nothing at about day 14-20.