Under English law just make sure your Will is easy to understand, correct and easy to find, those that witness the Will should not be beneficiaries, this will not invalidate the Will but it will invalidate the legacy, make sure the executors know they are executors (& probably beneficiaries). Name the three sons and put their current address, it will not matter if this changes but does help identification, it will also not matter if one is a stepson.
Remember, as bedknobs says, the survivor can change their will. If you are in any doubt consult a local solicitor.