Have a triplex but it is zoned as a duplex, what do I do?

[Lacey Nicole]

My grandfather passed down his property in Salt Lake City Utah. It is over 30 years old, but it was listed as a duplex, over the years he turned it into a triplex with a house in the back. Someone brought it to the zoning departments attention and we are going to be fined daily if we do not fix it. I do not know what to do, do I try to get the 3rd unit permitted? If so how can I do that after everything was done over 15 years ago?