If you want a triplet, click on a rest (a quarter rest for an 8th note triplet) and press Ctrl + 3. You can also press Ctrl + 4, 5, 6, 7 etc. for any tuplet, like a sextuplet or quintuplet. If you want a fast way to fill a whole measure with triplets, first split the whole note (in 4/4 time at least) in the measure into 4 quarter notes, and then highlight the measure and press Ctrl + 3.
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See: https://musescore.org/en/handbook/tuplets
If you want a triplet, click on a rest (a quarter rest for an 8th note triplet) and press Ctrl + 3. You can also press Ctrl + 4, 5, 6, 7 etc. for any tuplet, like a sextuplet or quintuplet. If you want a fast way to fill a whole measure with triplets, first split the whole note (in 4/4 time at least) in the measure into 4 quarter notes, and then highlight the measure and press Ctrl + 3.