Place mouse behind cue ball to set direction and speed. Press SPACE to shoot. Ball moves in direction of line. The white ball must hit the cushion 3 times and then hit both the red and yellow balls (in either order). You'll be told when you've hit a ball after 3 cushions. Keep playing until you miss, and try to make the highest run.
Converted game by @sennajnuet to rules of 3-cushion billiards. I'll add a cloud high score soon. For now you can post your highest run in the comments if it's better than the best posted so far... I'll start it off with a run of 4 :-)