Notes

See https://musescore.org/en/handbook/tuplets

You need to do this in several steps.

1] Make a triplet - three notes in the space of one quarter note:
5 [Ctrl]3

2] enter the first note - a “quarter note“ D

3] split the next note into two shorter notes:
click on it and press 3

4] enter the second and third notes

5] Split the next note into two smaller notes and then the first of those into two even shorter notes:
Press 3 and then press 2

6] Enter the notes

7] Insert the ties etc.