Many of you will have seen my recent videos around the Vigintuple cube - 20 cubes all attached together in the same manner as normal siamese cubes. There don't seem to be many tutorials on YouTube outlining how to solve a triple cube so I decided to make one. The outer cubes are just the same as solving a double cube so I won't be explaining how to solve them (go watch my double cube tutorial.
This tutorial will specifically go over solving the middle cube which can be quite difficult to figure out as the moves that can be made are very restricted due to having two 1x1x3 blocks attached together on the cube.
The solving method begins with using M and U moves (and the occasional R2 which moves the outer cube) to insert the bottom edges, then the cube is rotated and the outer cube repositioned to insert another 2 bottom edges. Afterwards, the corners can be corrected with a bit of messing around with the U layer and moving the R layer with the outer cube, once all the corners are solved, the rest of the edges can be solved with M slice OLL and PLL algorithms such as U perms, H perms, Z perms, OLL 28 and 57.
You can either make a triple cube yourself, or buy one from me here