The way F3A schedules have evolved, a triplane (and soon to be seen quadriplane with a twin cylinder gas engine up front) was inevitable. It is a straight forward way to minimize wing load. Consider the tripe has around 1500 squares and weighs 11 pounds. Loading is a scant 17 ounces per sq foot. Not quite foamy like but much closer. Also consider the quadriplane with it's 2200 square inch wing.....a wing load of 11 1/2 ounces per square foot. That's foamy territory
The dimensional limit up is 2 meters. The box is a 2 meter cube....unless FAI changes the rule (doubt that)