If it is not the active worksheet; you can mention the sheet name as well as
below
Sub Macro()
Workbooks.Open Cstr(Activeworkbook.Sheets("Sheet3").Range("a1"))
End Sub
If you are new to macros..
--Set the Security level to low/medium in (Tools|Macro|Security).
--From workbook launch VBE using short-key Alt+F11.
--From menu 'Insert' a module and paste the below code.
--Get back to Workbook.
--Run macro from Tools|Macro|Run <selected macro()>