The 1-3-1 was what led the University of Kentucky to many NCAA championships in the 1950s and 60s. Indeed, it does traditionally use 3-man to run the baseline to get to the corners. The one-guard plays on the strong side and the two-guard on top. The four plays the weak side and is expected to drop down to collect rebounds. Center stays in the middle. Though sometimes you play your smartest, athletic big man in the middle because he collects many steals.
It's a great, aggressive defense in real life that works very well until you encounter athletes that can play above the rim and go up for lob passes on the weak side. (This is why it worked so well back in the day then the game was played closer to the ground.)
All that being said, of course that may not be how BB has programmed it.