How do you do triplets on piano roll?
In the drop down menu for different quantisation subdivisions, (Looks like: "off (3840)" - just above the main piano roll window, right of the 'view' drop down, and left of the quantise button) select an option that is a multiple of 3 e.g, 1/3 note or 1/12 depending on how fast you want your triplets to be.