All three firms received overall scores of less than 60% as well as just two stars in the consumer group’s customer survey.
Scottish Power received the lowest overall score of 56%, receiving fewer points than any other provider when it came to its handling of complaints.
EDF came second to last with 58%, largely because its phone lines and customer service email accounts are not monitored for as long as other providers.
British Gas came third from bottom with an overall score of 59%, with Which? finding it offered a “good amount” of support to those who needed it but struggled with resolving complaints and switching customers.
E (Gas and Electricity) received the highest overall score of 79%, receiving four stars for customer service, ease of contacting it and the clarity of its statements.
Nearly nine in 10 E customers (86%) said they would recommend the provider.
Octopus Energy came second with a score of 74%, followed by 100Green and Sainsbury’s Energy with scores of 72% and 71% respectively.
Co-op Energy and Outfox Energy received the same overall score as Sainsbury’s Energy but did not score three stars or higher in every category of the customer survey.
Which? surveyed almost 12,000 energy customers during September and October for its annual customer satisfaction survey.
It also carried out an in-depth assessment of 17 energy firms’ behind-the-scenes practices and policies.
Which? Energy editor Emily Seymour said: “Our research shows that while four suppliers are outshining the competition, some big firms – including Scottish Power, EDF and British Gas – are letting their customers down.
“With fixed tariffs now reliably cheaper than the energy price cap, there is no need for customers to put up with poor value for money or shoddy customer service. Any suppliers falling short need to improve or their customers will vote with their feet.
“Anyone who is unhappy with their provider should shop around and move to a supplier with a cheaper fixed tariff and better customer service.”
A spokesman for Centrica, which owns British Gas, said: “Our investment in customer service is delivering results.
“Complaints have fallen over 20% each year for the past two years, and both Uswitch and UKCSI (UK Customer Satisfaction Index) have acknowledged the progress we’re making.
“We know there is always more to do, and we are continually looking at ways to make further improvements for our customers.”
It also highlighted efforts to extend its call centre opening hours and launch a dedicated line for vulnerable customers.
An EDF spokeswoman said: “EDF notes the findings of Which?’s latest survey and is committed to continuing to improve the service we provide to our customers.
“While the research highlights areas where we need to go further, it also reflects strengths in how we support customers who need additional help. This is consistent with feedback we see elsewhere, including our excellent Trustpilot rating of 4.8, based on more than 154,000 five-star reviews from customers.
“We are focused on building on these strengths and continuing our efforts to deliver improvements.”
A Scottish Power spokeswoman said: “The independent Citizens Advice scorecard is the customer service gold standard with a much more robust and transparent methodology used to assess firms’ performance.
“It shows sustained, high-quality delivery for our customers, reflecting the huge investments we’ve made improving and enhancing our service – also highlighted by customers themselves in the Which? survey.”
