Maintain 3 measure per row

If MuseScore is putting only two measures on a system, that would be because your current settings for staff size, music spacing, and minimum note distance will only allow that many measures. You;ll need to reduce one or more of those three settings. Staff size is in Layout / Page Settings ("staff space"); the others are in Style / General / Measure. If it's *close* to fitting right now, you could also try just squeezing those three measures a little without changing the settings globally - select them and press "{" (shortcut for Layout / Decrease Stretch).