Legal 2 unit with 3 units?

[David Goldstein]

I'm looking at a multi-family property in Providence, RI that is listed as "a legal 2 unit plus additional studio apartment" on the top floor.  Sounds like the studio has its own bathroom.  Plan would be to rent out both 2BR units, and I would use the studio when I'm in town every other week for work. 
Assuming AirBnB doesn't get outlawed in Providence, would I be able to rent out the studio on AirBNB for the days I'm not using it, even though it says legal 2 unit?