T3's problem is easily diagnosed. As home to Ryanair, FlyBe, Air France-KLM, American Airlines and Vueling it is massively over-subscribed. MAG has tried to persuade carriers to relocate but they won't ... they love it there. Proximity to the runways and all that. So the issues will remain for as long as passenger throughput continues to overwhelm design-capacity. Hence the problems with space in the airside departure lounge etc.
I have to say that whilst T3 security gets very busy I've had no problems with it. Allow time at predictably busy periods and you'll be fine. The queue-combing there is actually very well-organised in my experience. Passengers booked on more imminent flights are called forward and expedited through. Once airside (if early for your flight) I suggest looking for seating adjacent to the SE-facing window near the 50's gates. Always lots of space there. And a great view too.
If you fancy refreshments, I've found the 'Lion and Antelope' facility the most competitively priced eatery at any UK airside location I've come across.
T3 has its issues (where doesn't?). Mainly due to crowding. But it has its good points too. As a regular user of both Ryanair and FlyBe I'm in there frequently. It's not that bad! I do hope MAG find a way to expand its capacity though, particularly if all incumbent carriers continue to resist switching to another terminal.