Also, Busaboi doesn't call up the SPI.h library that I have seen.
The SPI library is very small. Busaboi just put the code he needed inline.
Assuming his code is good for 1 display (which I think has already been confirmed), simply changing the slave select from a constant to a variable should be all you need to get it to work for 3.
There's not a whole lot to understand with SPI: the really hard part is figuring out the "mode" (clock polarity and phasing), because SPI didn't start out as an official standard. So different chip makers did their own variations, and SPI never got formalized like I2C or USB. Sometimes you also need to be careful with the timing of slave select, or the pacing of transmitted bytes, but usually you can just go as fast as possible.
You've already got that right, so it's just a matter of stuffing bytes into the SPI transmit register in groups of 4 (and, in this case, ignoring the received data).