If it is a 4 sided pyramid then the green area will appear square when viewed from above.
You can make the 4 triangle faces by bisecting this square with the pyramid edge lines:
Each face of the pyramid is a right triangle with the right side at the top.
Draw your pyramid from above (square) with the top (green) section a smaller square centered in the larger one.
Bisect both with 45 degree lines (I drew a vertical line, copied it, pasted it, rotated new one 90 degrees, selected both, aligned horizontal and vertical, rotated 45 degrees and placed on my squares, then aligned Everything horizontal and vertical using Align to Artboard - with a square Artboard).
Apply a continuous pattern in the center square. When you are satisfied you can use the angled lines to divide the whole image with (Select All) Pathfinder-Divide or make a triangle that perfectly covers one of the faces and isolate just the one side for printing by using that triangle to make a clipping mask (Group All except new triangle, select group and new triangle, Left Click - Make Clipping Mask)
If it is a 3 sided pyramid the process is the same but with different shapes. Let me know if should illustrate that one.