three 8th notes in a quarter note value with "3" under the tie

Maybe read the Handbook? Specifically, look at "Tuplets"

This is called a "triplet". It is the most common example of a "tuplet", where you can play any number of notes in any duration. See https://musescore.org/en/handbook/4/tuplets. Here are some examples; the first three are "triplets"; the others "tuplets" with different divisions.

You will sometimes see tuplets without the brace and even occasionally without the numeral. You can set either of these settings in the Properties tab, as described in that Handbook entry.

I recommend to all new users: read the manual! Take an hour or two and read through the entire thing! You won't learn everything about it, but you will know what is present and (roughly) where to find it.