The plans for T3 are actually bigger and more ambitious than both the T1 & T2 redevelopments put together. I have detailed a few snipets in previous posts if you want to try and find them but at this stage there are several possibilities being looked at and of course any proposed scheme has to be put forward for board approval.
The current Mezzanine linking T1 and T3 (where the Donkey Stone pub was located) has been closed for a while now. This will not become part of T1 airside for the time being. The T3 end will be made into an extended security area which will serve all passengers departing from T3. Some space will be left at the T1 end with the idea of creating new executive lounges at a later date. There is talk at the minute that the airport's own 'Escape Lounge' brand, currently only in T2, will open a new lounge in T1.
As for the link between T1 and T3, this unfortunately will now require passengers to walk outside between the 2 terminals. A new undercover walkway will be built between the Thomas Cook lifts and the Spar entrance at T3. This is only a short term (2-3 years) measure and eventually a new skylink will be built between the station and T3 but not before the new plans for T3 have been finalised. There was initially going to be a shuttle bus between T1 and T3 but plans for this were shelved as the airport decided the added congestion on Tower Road with these additional bus services wouldn't be worth it for what is effectively a five minute walk.