Why can't I save file with multiple sheets instead of only the last active sheet I was in?

I opened csv file as spreadsheet.
Sheet1 is listed.
Copied a few lines from sheet1 to sheet2.
Copied a few lines from sheet1 to sheet3.
Saved file while in sheet3.
Closed file while in sheet3.
Re-opened file .
—Only Sheet1 now but it’s the data from sheet3 the last one I was active in.
The original sheet1 and sheet2 are gone!!!

Because you can’t save more than one sheet in a CSV file. Take in account that CSV files are plain text files not spreadsheet files.

If you want save all sheets use Menu/File Save as / and select as File type ODF Spreadsheet (.ods *.ods).

You can import a CSV file in a spreadsheet with Menu/Insert/Sheet and if you like retaining it linked, checking the option.