@Shantaviae Wynn - as stated above and to be clear.. PROCEED WITH CAUTION.
As MaxT said.. RSA5 is usually single family, but can have a variance (I owned a duplex recently that was setup in this manner).
Long story short, to be absolutely safe I would go to L&I to obtain the actual zoning records and certificate of occupancy. Confirm via hard documentation (from zoning) that it is approved for USE as a 3 unit building. Doesn't matter if it's RSA5, etc... if it's not "Approved" for use as a 3 unit building, you may run into trouble down the road. And I wouldn't take someone's word for it over the phone.. get it in writing from the city. It's worth the fees (should run about $100 or so) for the paperwork.
I have a friend who purchased a duplex 2 years ago.. after closing found out that RSA5 was not "approved" for use as a duplex in Passyunk Square neighborhood (no variance present). He had to hire an attorney, go to zoning meetings, etc to obtain a variance to use as a duplex (he claimed by right since meters are separate for each unit). HOWEVER.. the local RCO didn't like the duplex idea and denied him. Luckily he had a good attorney who fought with the city and eventually won the variance...but that was a good year long ordeal.