In Libre Calc, when I try to insert a row, I get the message, "Filled cells cannot be shifted beyond the sheet." This message makes me think that the file "thinks" there is data way out to "infinity" in this spreadsheet. In actuality, I'm only using three columns which easily fit on a standard 8.5x11 inch paper. How do I fix the spreadsheet so I can insert rows?
You've said you only have three columns. But your error relates to rows, not to columns.
I could reproduce your error by going to the bottom-most row, row 1048576, of the spreadsheet and typing something in a cell in that row. After that, if I tried to insert even one row anywhere in the sheet, I got the error. Similarly, if I filled a cell in row 1048566, and try to add eleven rows above the row in which I entered data, I get the error. I don't get the error if I insert only nine rows or ten rows above that row. Note how the row with the filled cell is pushed down to exactly the end of the sheet when you add ten rows.
In other words, your sheet can have only 1048576 rows. If you work at the top of the spreadsheet, as most of us normally do, we don't notice the limitation of 1048576 rows.