For a triplet comprised of 3 quarter notes played over 2 beats, select a half rest - i.e., the total duration of the triplet - then press Ctrl+3. You will get a triplet of 3 quarter rests, which you then can fill with notes.
See the picture here for examples: https://musescore.org/en/handbook/3/tuplets#Triplet_examples
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For a triplet comprised of 3 quarter notes played over 2 beats, select a half rest - i.e., the total duration of the triplet - then press Ctrl+3. You will get a triplet of 3 quarter rests, which you then can fill with notes.
See the picture here for examples:
https://musescore.org/en/handbook/3/tuplets#Triplet_examples