"Triplex" with Non-Conforming 3rd Unit

[Eli Sunderland]

Hey everyone! 

I found a property that is labelled as a triplex and zoned as a duplex, but has a non-conforming 3rd unit that the owner has been renting out.  My question to you is, how do these kinds of properties work?  I don't want to buy it as a triplex, then be unable to legally rent out the third unit.  

Do any of you have experience in this kind of situation?  

Thanks!