What exactly is the question?
The original question was:
is this a possible 2nd or 3rd plane. The answer is NO this is not a good second plane, 3rd plane? Maybe, depends on your skill at that point.
The 3D question? The Sundowner is not a 3D plane. 3D planes have large ailerons that run all the way to the fuselage so you can fly the plane in stall conditions utilizing prop wash. The Sundowners ailerons aren't big enough or close enough to the prop wash to have any effect.
Aerobatic? to a point, 3D? NO.
Make sure you know what the difference between aerobatics and 3D are.