Large CSV File (Multi-Tabs) - Multi Sheet

I was merging 3 large files CSV with roughly 4000 rows and 600 columns

I copied all three documents into one WorkBook - so I had 3 tabs and could just bounce between them.

After a full days work, I went to save and it would not save the work and all it said was it could only save 1 sheet.

I wasnt sure what was going and I do not know how to save my entire workbook.

Can you tell me how to do that please? I have lost all my work and need all these together

Any input?

Save your file as .ODS file which can handle many sheets, formatting, formulae etc.

A CSV file can by definition save only 1 sheet - Please follow the advice of @Rugslug.

Repeating the comment by @Rugslug (credits to him/her) as answer:

You are attempting to save a multi-sheet file as .csv (probably stemming from importing one .csv file), save as .ods instead.