So, here is your tutorial on how to create a triplet in MuseScore. You can see that I have a triplet created here, triplet quarter notes. These three quarter notes are in the value of a half note. Normally there would be 1, 2 quarter notes in that value of the half note. I want to get a triplet over here.

What I need to do is to turn on your notes, select the value you want, get into the position in the measure you want, then press control and the number 3. See that created the space for me to put my notes in, and then I can add my quarter notes how I wish. That takes care of how to add the triplets.