Why aren't 3-surface aircraft more popular in general aviation?
Canards are not used in general aviation simply because they have complexe interaction with the main wing which can leeds to issues with stability and being at the front of the CG their effect on flight dynamics may be highly unintuitive in some situations. Therefore you need electronic control to adapt the pilot input to the control surface in order for the plane to fly as expected, and that's why in my opinion it was not suited to GA where electronic was expensive until the past few years. But they are plenty of jet fighter using them as they have the ability to greatly increase performance and maneuvrability.
In addition 3 surface aircraft means more wetted aera thus more drag, complex design and control, more mechanical moving parts which will often drive the price up.
You can find more information looking at the patent issued in 1988 by Piaggio here : https://worldwide.espacenet.com/publicationDetails/description?CC=US&amp;NR=4746081A&amp;KC=A&amp;FT=D&amp;ND=&amp;date=19880524&amp;DB=&amp;locale=en_EP