I cant ever see T3 going low cost.
Their business model is and always has been to price fares against the first class return rail far and market it on the basis of if you had a days business to conduct, how long it would take to travel by road or rail there and back and how much time would that leave you compared to a an air service which got you there first thing and allowed you to depart as late as possible, still allowing you to be home and refreshed at a reasonable hour.
It seeems to have worked for them so far and suspect it will for a long time to come, mainly because the low cost airlines cannot afford to operate from the majority of airports at the times which Eastern do.