The I9 must be completed within 3 days. I see nothing in the law that allows an employee that works less than 3 days to not have an I9.

EDIT: A look at the M-274 I9 Handbook for Employers https://www.uscis.gov/sites/default/...form/m-274.pdf

It says on page 7, 3rd paragraph...

If you hire a person for fewer than three business days,
Sections 1 and 2 of Form I-9 must be fully completed at
the time of hire – in other words, by the first day employment
for pay begins .