Same experience here.
Currently on rolling contract 15 euro a month with 3.
Told them I got my sim from gomo (which I have) and wanted to check if they could price match.
They told me best they could do was 20 a month. RE-iterated I was already on 15 a month so if that was the best they could do I'd just move to gomo and asked how could I cancel with 3
Then told me they'd chat to the supervisor ...back a minute later with 10 euro offer effective 15th November.
All completed in about 10 mins on webchat
Very happy bunny...cheers OP for giving me the push to ask!