Hi Derek. So I don't own anything in the city of Pittsburgh but do have rental properties in the surrounding counties of Westmoreland, Fayette and Washington. I would be really concerned that it is a triplex but only zoned for a duplex. So it sounds like it is unpermitted. Not sure about how the city of Pittsburgh would work but it may be really difficult to get the zoning changed. You would need to ensure that there is adequate parking so I am sure that is one thing the zoning officials will be looking for.
Probably most concerning is the fact that if the city would approve it they would probably require a sprinkler system be installed because of the third unit. I ran into this in Fayette County with a similar situation. It was a duplex that we wanted to change to a triplex. It was approved by the county but the sprinkler system was a deal killer so we didn't go through with it.
If you do end up buying it and need legal advice I would recommend Attorney Brad Dornish. He is very active in the real estate group Acre of Pittsburgh and is very knowledgeable in real estate and landlord/tenant issues.