TCX
Based entirely on their schedules, which I can see do need some re-jigging, it seems a 3rd aircraft comes down from Glasgow from mid July until the end of August (in addition to the A330 which also comes down from GLA, actually making STN a total of 4).
I wasn't sure if possibly a 757 might position into STN to operate Skiathos which is one of the Friday morning departures. It seems to be operating non-stop in 2016 (currently the return leg goes via Kavala as the A321 can't make it direct to the UK).
Nonetheless I expect it's more likely the schedules will be changed but definitely looks as if 3 A321's will be needed, even if/when schedules do change.