A few years ago, I designed and marketed a couple of sport scale triplane kits, the Fokker DR-I and the Sopwith. Both had flat bottom wings, 3 degrees of right thrust, 3 degrees of downthrust and the incidence was set as follows:

Bottom wing 0 degrees to the horizontal
Middle wing -3/4 degrees to the horizontal
Top wing -1 1/2 degrees to the horizontal

Both flew great and had no bad habits.

My 2 cents worth...

Jake

PS,
Here is a photo of the last tripe that I built from one of my kits...