Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
Programs: BA Lifetime Gold; Flying Blue Life Platinum; LH Sen.; Hilton Diamond; Kemal Kebabs Prized Customer
Posts: 63,821
You can't just leave T5 and enter T3 like that, a T5 boarding pass won't get entry to T3 via landside. You have to use the transits to T5C, come back to T5A, go airside to T3, clear security, return via airside transfar, renegotiate your Ready to Fly Status, hope you keep your seats in the process and then reclear security. Mostly you would be mad to do this, when there are many easier options open to you.