What you can certainly do is ride the bus to the T3 building and back again, but I agree it's not worth the flapping about, you treat yourself to at least one superfluous security check at T5 on the return, and another if you are able to get through T3 too. The boarding pass checks (apart from at T5) was always negotiable, you could say you were going shopping in T3, for example. If you do your adios in T5 - which is a nicer area anyway - then you can go airside from the EDI service without security.