Three in the UK was always a rather small network. Arguably most money was put into building the network and sustaining demand, which it has done very well, but it has never been able to make the big league.
Hence issues with retailers, presumably because of a lack of margin. Hence issues with basically not getting many business customers (do Three even do business sales anymore?). Hence the lack of handset choice.. etc
Three also undersold itself, perhaps out of desperation, to get subscribers. When it discounted the One Plan to £15 a month on SIM only, it was ridiculously cheap. I, and loads of others, took advantage of it (well, why not?) but let's just pause to think how mad it was.
Besides people going over their minutes (unlikely; people make less calls these days) or SMS allowances (probably unlikely too thanks to IM), you had no way to make extra money from customers as you offered AYCE data and tethering. And of course some people took advantage of it (and can't be blamed IMO) by using tethering to the full.
I can see why Feel at Home needs to be restricted, but I don't agree with it. Especially when it's being sold in a way as to suggest it's just like being at home. I'd have fully supported something like 1GB per day (or whatever) at full speed, then capped, or perhaps limited to Email, social media and you pay extra for full access - with fixed allowances purchased on a daily/weekly basis, with no restriction (and with tethering, as you've paid for a fixed allowance).
This year Three has done lots of things that totally go against its 'make it right' campaign. Introducing RPI increases, reducing the Feel at Home allowance from 25GB to 12GB (I only found out when they contacted me over my story, and it was said as if it was ALWAYS 12GB) and other little price hikes, PAYG add-ons being tweaked (not in favour of the customer) and so on.
I think Three needs the O2 purchase to go through. It's a fantastic network, but once it kicks everyone off the One Plan, keeps increasing tariffs to the point where they're not really that competitive and Vodafone and O2 have caught up in terms of coverage and capacity (likely from end of 2017 onwards we will see a massive transformation), will it be a network that people feel they need to go on?
You'll have EE at the top, for sure, and Three may well find itself falling beneath O2 and Vodafone and becoming almost irrelevant.
What's odd is that the parent company wants Three to become a big league player, but the marketing is still trying to give the general public this idyllic vision that Three is the small guy 'sticking it to the man'.