I'm no so sure the lower wing drag won't cancel out the upper wing drag. The overall shape of the "stuff" seems to put the center of drag for the wing stack at the middle wing or pretty close to it.

To best evaluate the ability of the tail to contol all that "stuff" you need to do a tail volume coefficient calculation but I'm not sure how to put all that area into the figureing since the area AND chord are part of the calculation. I'm going to guess that you need to treat it as a short chord wing with a HUGE span to get the area and ignore the fact that the area is folded over each other in that triplane configuration.

There is some tail volume coefficient calculators out there that use Xcel spreadsheet or you can do it manually at a lot of other sites. I had some links but they went away a couple of days ago when my hard drive died. Do a google search for tail volume coefficient calculation or calculator to see if you can find them. Most of the sites with the TVC info also include what the desireable range of the value should be.