Input: the green shaft to which 3 gears (green and cyan) are fixed. Output: the blue shaft on which a block of 3 gears (red, yellow and orange) can slide (key sliding joint).
Turn the pink lever to get 3 positions of the gear block corresponding 3 speeds of the output. There is a positioning device (spring ball) in the pink crank.
To avoid gear collision the speed change must be carried out when the input shaft stops. The operation shown in this video is for the case of very slow speed.