One possible way would be to copy sheet 3 to a different Workbook and save that.
For instance, on the Workbook.BeforeSave event, you could have some code that copies sheet3 to a new workbook and saves it in a certain place, and then cancels the save event. On the Workbook.Open event you could have code which opens the other workbook and copies the contents into sheet3.
If you just have simple text on sheet3, you could alternatively write the contents to a text file whenever sheet3 is deactivated, and read the information in from the same text file each time it is activated. this is surprisingly fast as long as you are not reading in War and Peace!