I don't like the sound of that?! Can you elaborate at all?
I'm a mostly happy Three customer: good value, mostly reliable, Roam Like Home (which no other network offers, afaik?).
But: their webshite is abysmal, their customer account admin area limited (it will only let you switch your tariff to other, now poor value, tariffs as existed at the time that you last switched tariff, rather than current ones).
Rather than battle with their contact centre (don't worry, I tar all of them with the same brush, it's a very inefficient way to ask a computer to update your database record for you), I found it easier and quicker last time just to request a PAC by SMS, sign up for a new SIM-only tariff elsewhere, port my number, and then repeat the process after a month. No poor quality voice calls with some poor call centre worker with mutual accent comprehension difficulties needed.
Computer says Yes!
Just text PAC to 65075 and you're ready to port!
(Ofcom definitely do deserve credit for introducing this.)