Sorry didn't specify where to put the code.
Put the code into your workbook_open event so the code will look like this:
'change the sheet1 to your sheet name that contains the combobox
'change the combobox1 to your combobox name if different
'change Sheet2 & 3 name to yours
'for reference Cells(7, 5) is equal to Cell E7 on Sheet2
Code:
Private Sub Workbook_Open()
Sheet1.ComboBox1.AddItem Sheet2.Cells(7, 5).Value
Sheet1.ComboBox1.AddItem Sheet3.Cells(6, 5).Value
End Sub