one way:

Put something like this in the ThisWorkbook code module:

   Private Sub Workbook_BeforeSave( _
               ByVal SaveAsUI As Boolean, Cancel As Boolean)
       If Application.CountA(Sheets(1).Range("A1,B2,C3")) <> 3 Then
           Cancel = True
           MsgBox "You must have entries in A1, B2, and C3 in Sheet1"
       End If
   End Sub

If you're not familiar with macros, see

   http://www.mvps.org/dmcritchie/excel/getstarted.htm