What is the best design to stack wings of a triplane?
Wing stagger was mostly chosen to improve pilot vision: In single-seaters, a positive stagger would allow to place the upper wing ahead of the pilot, improving the field of view.

Large bi- and triplanes had no stagger. Here the pilot sits ahead of the wing and large aircraft fly mostly straight, reducing the need for the pilot to observe the airspace around him.

However, if you look for a proven design of a very compact triplane, I would like to suggest the Italian Pensuti triplane. It had a wingspan of only 4 meters and a total weight of just 230 kg. Emilio Pensuti, a test pilot with Caproni during WW I, designed it as an easy to fly aircraft for the common man. Therefore, it is sometimes also called the Caproni-Pensuti triplane. The first version was powered by a 35 HP Anzani engine while a later development, the Breda-Pensuti 2, used a 80 HP engine.

Of course, its wings had no stagger.



Drawing of the Pensuti triplane (picture source). Yes, it did fly!



Pensuti triplane in flight (picture source)

If you build one, I would strongly suggest to change the wing airfoil to a thicker one with the same amount of camber.