notations

First you have to select the good lenght of your future triplets.

Exemple : if you want a triplet of 3 eight notes, his lenght will be 1 time . So you must start with a note or a rest of ................1 time (using the shortcut "5" . If you have, for ex a whole rest , clic on it, tape 5 , you get a rest of 1 time......When you get the good lenght of 1 time , clic on it ,do CTRL+3 and you get your triplet https://musescore.org/en/handbook/4/tuplets