If it's marked Fairport instead of Rochester, it's probably from the 1920s, since Gundlach moved around that time frame. It's a triple convertible as you guessed, and will cover 5x8 (as it states). If there isn't too much separation in the balsam, they're nice lenses - if you look at the reflections, you'll see that it's 5 cemented pieces of glass per lens element. Combined, you get 8 1/2" FL, just the rear gives you 14", and the front gives you 20". $85 is probably a decent price for one of that size (assuming minimal separation and a working shutter).
Dan