The only thing I have Three for is data, as the only other people who do a decent mobile broadband allowance are Vodafone and they're too stupid to post me a SIM when I ask for it, but then won't let me sign up with my details again to request another.
And every time Three adds incentives to stop people using traditional services and "use your data instead", it suggests that they need to increase their data allowances even further.
But then, I only ever do month-to-month contracts, because I can then just swap out the SIM any time I like without arguing over what is a "material change to the contract".
Honestly, though... if someone sells a 100Gb 4G SIM on pay-monthly that allows tethering for that full amount... someone shout, because I haven't found one. The 100Gb packages on Virgin don't say they allow tethering and their own "mobile broadband" SIMs only go up to 10Gb. Vodafone does a 50Gb one but are incompetent. Three do 40Gb. Everyone else (Giffgaff, O2, EE, etc.) has silly "unlimited (***********)" clauses or don't allow the full amount of data for tethering (which is what mobile broadband is considered as).