Using an FHA Loan on a questionably permitted 3-Unit?

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I'm looking at putting an offer in on a property utilizing an FHA loan. The property is on the market as a 3-unit, has 3 renters currently in it and probably has for a long time. The area however is zoned single family and only one unit appears on the tax record. I've toured the property and it's in great condition. Separations were clearly made over the life of the property but everything appears to be in sound order. There is an unpermitted bedroom addition in one of the units and there are three full kitchens on the property.

I need the FHA loan up to the 3 unit lending limit to really have chance at this and I think it will come down to the appraisal. What will the unit count in the appraisal be based on?