Utter shite. It Varys from area to area... I’ve been with Three for donkeys years and in most areas I go it has outperformed O2 and Voda for overall coverage and quality, especially 3G/4G.
As you personally know, I have in recent times been with O2, Voda, EE and Three, and I do get about a bit. I actually still have live SIMs with all but Voda (and my O2 is via MVNO NiffNaff). Voda is dreadful in these parts, on a par with Three.
Generally, Three is the worse for signal coverage by some margin. O2 the best for voice coverage (but probably the worse (usable) data coverage).
For 4G coverage, there is only one clear winner - EE. For 2 reasons, it was first to market and was keen to capitalise on that, and its also the only one with a legal obligation to provide a set geographic coverage value (all have a 90% outdoor population (which is lower than geographic) as part of their 4G licencing, and all but EE are missing that).
I have multiple SIMs from different networks, because I always need data, for those situations where I need to get on the network (be that work, bro's, or home) quickly from the nearest layby (I need DTM's Jag's wifi
). I'd have to say, since moving my personal mobile to EE 2 or 3 years back, its been a revelation, as I'm struggling to think of any time where EE failed, and the NiffNaff or Three SIM worked. In fact its rare when outdoors, my gayPhone isn't reporting a decent 4G signal.
Local coverage will always vary, as I said in my original comment, and as henryd pointed out that local to him Three was superior. When we stay in Cornwall, its usually in the rural areas around Hayle/Helston, and we've frequently camped near Leadstown, down in the valley, where nothing works... ...thankfully the campsite put up a load of BT Wifi points
A good pal, Terry the Chipman, uses Three on his Samsung Galaxy, and even he will agree the coverage is shocking. It works, just, where he lives, doesn't work where he works, and only has intermittent coverage in the 17 mile commute - something we always laugh about. But, he's on a legacy contract, unlimited (untetherable) data, unlimited mins, unlimited texts for under £20, and he doesn't want to give that up, understandably. He just hopes Three will improve coverage... ...but he's been saying that for the 10yrs I've known him, with barely any improvement.
Obviously, beyond signal, be it 2G, 3G or 4G, there is an issue of capacity. Milton Keynes is particularly bad in this respect, with all networks mostly offering a good signal outdoors around most of the borough, as you would expect. But its quite common not to be able ot even make calls or send texts when something happens, like heavy snow at rush hour, due to capacity.