Tuplet
Tuplets are used to write rhythms beyond the beat divisions usually permitted by the time signature. For example, triplet eighth notes (quavers) in a 4-4 time signature divide the quarter note (crotchet) beat into three instead of two.
Instructions
To create a triplet first select a note on the score that specifies the full duration of the triplet group. For example, a group of triplet eighth notes (quavers) has a "full duration" of one quarter note (crotchet).

From the main menu choose Notes → Tuplets → Triplet. This creates a triplet by dividing the full duration into three equal parts,

which can be further edited.
Note entry mode
Tuplet entry works a little differently in note entry mode than the method outlined above. In version 0.9.5 or later you must select duration first then enter the pitches afterward. Below are step-by-step instructions for making triplet eighth notes.
- Switch to note entry mode by pressing N
- Make sure the note entry marker is in the place that you want to start the tuplet (use the right and left arrow keys if necessary)
- Choose the duration for the whole tuplet group from the note entry toolbar. For this example click on the quarter note (or press 5 on the keyboard)
- From the main menu choose Notes → Tuplets → Triplet, or press Ctrl+3 (Mac: ⌘+3)
- Notice that an eighth note duration is automatically selected. Click on the staff to add pitches
Properties
To change the display properties of a tuplet right click on the tuplet number and choose Tuplet Properties....

The number section of the properties dialog allows you to show one integer, a ratio of two integers, or no number at all.
In the bracket section the "bracket" and "nothing" allow you to explicitly show or hide the bracket. The "auto bracket" option hides the bracket for beamed notes and shows the bracket if the tuplet includes unbeamed notes or rests.

External links
- Tuplet at Wikipedia
- How To Create Triplets in MuseScore [video]