We don't have the ability to export directly to Excel, so I don't know about this, but I do know that Excel will often open a new workbook with multiple sheets - default tends to be three unless you change that.
When you say you only get the sheets when spreadsheet already exists, does that mean when you originally create the spreadsheet via SAS, you don't get it, but then you Replace into it, it adds three blank sheets? Or do you mean you've created an workbook via Excel and then export into it?