Rudder size?
As I am not sure as to how big it is now, or the length of the fuse, it is difficult to say.
Adding the center engine helps. The problem with wing mounted engines, is that the rudder is not in the air blast to help with the steering work.
On my twin ugly sticks, I increased them 50%. My bashed Ultra Sport 1000, I doubled it.
The reason to want, or need, to enlarge the rudder is that the loss of one engine creates yaw, and you want to be able to counter it as much as possible.
Good Luck, sounds neat!!!
The Kadet that one of the guys at my field has, that I caused him to "Twin" and then caused him to "tri", actually flies better as a tri than it does as a twin.
OK, so when we "Caused" him to go to five,,,,,,,,that was a bit much.
Twinman