Trying to create multiple exercises on the same page with the same measure width

Hi. In this case, I would make my section break every two measures.

Insert a horizontal frame after that measure and apply the section break to the frame rather than the measure.
You can then resize the frame as desired. Enlarging it will shrink/squeeze the measure before it.

The problem is that you have a section break after a single measure. Delete that section break and continue. To delete a section break, select, right click and delete. Or you could simply select that last measure and insert another measure. That will leave you with two measures before your last section break.

If you want three measures per line then place your section breaks in the right places, at the end of every three measures. Delete any section breaks that are misplaced and continue.

Musescore doesn't know how to size the measures until you place your section breaks. So you get these odd looking tail measures. I usually plan the page a little bit. In your case, you might be better off with 8 measures to a section. Add a blank or warm up measure to each group of exercises. It will be easier for your layout to come out nicely. Or you might try three measures, section break, then four measure, section break, to avoid a hanging tail measure. If there is too much stuff in four measure, it won't fit and look nice, so try laying this out as 8 measure sections. Make up an 8th exercise or add a warmup measure.