Zoning and Land use single fam, Duplex, Triplex?!

[Jeff Prather]

I'm analyzing a deal in Nashville, TN that was used by the seller as follows. He  lived in unit one and air bnb-ed unit 1's "in law" suite. Unit two was on an annual lease. 

I'd like to buy the property and live in unit 1 and rent the "in-law" suite and unit 2 on annual leases. The in law suite is fully functional as a stand alone unit (kitchen, bedroom, living room, bathroom) and is separated from the unit 1 by an interior door that locks and has its own exterior entry. Basically the property setup is that of a tirplex. 

The zoning is RS10 - in Nashville this is single family zoning. The land use is residential duplex on the property assessor records meaning the property is legally grandfathered as a duplex as it has two separate utility records for gas and electric historically. 

The numbers fit my criteria when I run this as a triplex on 3 annual leases - this is what I intend to do with the unit once I move on from living there in a few years.  They are more break even if I am legally forced to rent it as two units. 

My question is: what are the rules and enforcement for using the property as a triplex. Is anyone else out there renting out properties in a similar situation? Who would be enforcing the dwelling use (zoning, codes, ect)? What is the likelihood this would get on their radar and I would be disallowed from using the property as a triplex?