Would be very surprised if T3 entertain STN, I used to fly their LCY route on business and it was hugely expensive (£350+ return). STN is typically used for either connections to cheap flights elsewhere or as a cheaper means of getting to London. Now working with EZY prices at around £50 return with the £30 on top for the Stansted express that just about competes with the LHR and LGW offerings.
So, if T3 come in with their £100+ offering to STN I cant see it being viable in the long term. Especially when you have cheaper means of getting to the same place from other carriers. If FR don't step in then it may be a total loss?