This setup has a large center box fin on the stringer, with a small fin either side called side bites.
Found on most performance orientated longboards and some min-mals & midlengths the 2+1 gives you the smooth transitional surfing of the single fin with a little added hold.
The addition of the side fins (or side bites) allows you to “bite” into the wave face allowing you to surf your bigger board more top to bottom on the wave.