Three are exactly where they deserve to be, the merger was a total none starter soon as anyone with an grain of sense looked at it, they'd maybe have had more luck trying to buy EE from DT, and BT buying O2. three would have had a state of the art network and BT could have used its access to openreach to help roll out those much needed rural mast upgrades which judging by the Vodafone predicted coverage map are getting scaled back more each week, maybe due to an unwillingness to use BT fibre for its networks..
Also three may not be swimming in spectrum like Vodafone & EE, but it's not that bad off compared to O2, it's just they've chosen to do bugger all with it!
15x1800mhz & 5x800mhz 4g plus there chunk of 2100mhz 3g should give them good speeds for a few years yet, plus aren't they due to get given some other 1400mhz spectrum? If they'd been arsed roll it out they could have had 1800mhz on most masts by now, along with a Base layer of 800mhz and God forbid been inventive like O2 had been, and refarmed some of there 3g spectrum to support 4g in highload areas. I don't think things would have been that bad.
As for competition..
At the present time, things are OK, but give it a few years and I'm not so sure.. Forgetting the historic reputation BT has on future investment for a second, presently and from what I can see to be the medium term, if you want truly mobile data communications, EE is and will be your only real option weather it be for speed or reliability, cornerstone are years away from this point at present rollout speeds and that's even before you take into account that they are hamstrung keeping 4g on lower power due to no adequate call fallback. Plus many 4g speeds are now sub 3g speeds.
That leaves EE fairly unrestrained when it comes to the cost of plans and the ratio of data you get for your bucks..
12 months ago I had a unlimited plan which cost me around £20 with three, I now have a 20gb plan with Vodafone costing me £30, in order to actually be able to use some data when I need to I'm looking at EE who only do 15gb max sim only for roughly the same cost..
I think we'll see these data Costs rise and rise over the next few years.