If they want to leave sooner allow it with no penalties, you just want possession back.Be reasonable in what you expect and what of the deposit you keep. 

A "child" LLC is cheaper to establish than a new "parent".Ask for legal and financial professionals for their input.Starting out, you just want to protect your personal possessions of the inherent risks of the business.

Can I just do a 3 day notice, change the locks and take possession? 

Partial payments received after legal proceedings are commenced do not constitute a waiver of the right to obtain possession and terminate the lease agreement.

He took possession of the keys which makes him responsible for their safeguarding.

It would be extremely rare to find a seller who will give you possession and title without payment.

Florida is actually a landlord friendly state if you are only looking for possession of the property and no back rent or other damages.

For example, a landlord cannot use threats of violence, remove a tenant’s possessions, lock the tenant out of the apartment, or willfully discontinue essential services such as water or heat.

Evicts are styled:Previous Owner(Plaintiff) VS Current Tenant(Defendent) whereas the suit is for past due rent and possession of the property. 

It is also important to keep most utilities on for the turnover and showing of the unit.Specific to Washington State: The final report on accounting for return/withholding of deposits must be made no later than 14 days after the tenant has vacated and the landlord has regained legal possession of the unit.