@Adeeb K.,
Is there an ACTIVE rental license for three units? If so, you should be fine. If the rental licence has lapsed, then you may have a problem. There is a grace period, but you would have to check with the city on that particular. If there is no rental licence, you will have to go through a long zoning appeal process, then most likely have to file an appeal with the Board Of Building Standards because i'm assuming there is no sprinkler in there, which is usually not an issue with existing triplexes, but if you're changing the use from single family to triplex, it will trigger a sprinkler requirement, among others. All this can, and has been done, but takes time and money.
My advice would be to check the status of the rental license first through Eclipse system, or emailing the city, or asking the owner to provide proof of active rental license for 3 units. If you send me the address, I'd be happy to look it up for you.
If there is no license, perhaps you could negotiate a zoning contingency in which the sale is contingent on getting the use. If you're doing a total gut rehab, I would just factor in the cost of the sprinkler system.
Hope this helps