The airlines are going to have to do some comms on this. I have a Ryanair itinerary next month that specifically states T3 for departure. I can already foresee plenty of people getting dropped off at T3 only to find security shut and sent back outside to T1 (probably to join the back of a long queue). What a mess.
Terminal 1 is only a quick walk, through a covered outside walkway, fromTerminal 3. It’s not the end of the world if people are dropped at terminal 3.
The main issue is going to be the security queues in Terminal 1, but clearly the airport doesn’t have enough security staff so opening T3 for security departures wouldn’t help. The same number of security staff across the two terminals.