Composer, teacher at Berklee College of Music in Boston. Finale 26, Mac OS X 10.14
Hi Les, I think what you need to do is: Select Speedy Entry. Make sure "Jump to Next Measure" is deselected. Select the measure where you want to insert the first three beats, positioning the note ...
There's a JW Plugin that handles cadenzas brilliantly, taking care of spacing issues that occur if you simply hide barlines. It's called JW Conceal Barlines.