Do I need proof of service of the 3 day notice to vacate? Is posting the form at the property legal if the tenant isn't available? The property is located in Polk county, Florida.

According to Section 83.56(4) of the Florida statute, "The delivery of the written notices required by subsections (1), (2), and (3) shall be by mailing or delivery of a true copy thereof or, if the tenant is absent from the premises, by leaving a copy thereof at the residence."

No, you don't need proof of service for the 3-day notice. You will need proof of service if you file a lawsuit for eviction, however.