Is the Tenshinhan's Shin Kikoho technique, a triangle or square?
Though he put his hand in the form of triangle, the hole is in square/rectangular form. The answer is quite scientific. A Reuleaux triangle can be rotated to create a square shape. This YouTube video and this site explain it in more detail.

Tenshinhan's hand shape is similar to the Reuleaux triangle. Due to its shape, while it rotates, its cutting edge cut the hole in a shape of square which is shown in the gif below.