3 weeks would be useful, doctors try to avoid doing labs because of costs and insurance issues.
E2 levels may take time to evolve, But an earlier lab is still useful.
If you start on AI at first, first lab provided ability for calculated AI dose change.
SHBG and FT may have a time evolution.
PSA check at 3 weeks probably useless, wait for later labs.
CBC and hematocrit may take time and could be second labs.