Triplets in Musescore 4.5.1

" It creates the triplet, but just the first note is C...the other two are rests. How do I get three C's?"
Well, by completing the triplet with the entry of two other C's

In the handbook: https://musescore.org/en/handbook/4/tuplets

In @cadiz1's link you will find that "select the note the triplet is based on" means to select the duration that the triplet is based on, not the pitch. So, if you want to make a tuplet (the generic term that a "triplet" is a specific instance of) that lasts the same time as two beats, you need to select a half note, or half rest, and then press Ctrl+the number of notes that duration is to be divided into. This creates that number of rests over that duration that you can then enter notes over.

Two examples:

a) You want one beat (in 4/4) divided into three notes. Select a quarter rest, or a quarter note, and press Ctrl+3. You will get the first of the two examples below.

b) You want an entire measure (in 6/8) divided into five notes (instead of six). Select the "whole measure rest" and press Ctrl+5. You will get the second of the two examples above.