3 year fixed electricity charge - good deal or what?

my electricity company (edf) have written saying for an extra 3.47 a month they will fix my electricity charges for the next 3 years - has anyone else had the same sort of offer and if you have, what do you think? i will end up paying over £40 a year more for electrcity which i think is ironic - paying more to stop paying more?! i also think once the 3 years are up they can just wack on one big hike. any thoughts?