The watchdog is concerned that if the outfits' auto-renewal policies are fair, how easy it is to get a refund (or cancellation) and if the terms of service grant these companies too much power.
The investigation is part of a "super-complaint" on the loyalty penalty, or the tendency for some companies - not just in gaming - to punish long-serving customers by making it difficult or costly to leave.
Officials emphasised that the investigation had only started and that it might only take "enforcement action" if it believed the companies weren't being fair.
Sony has raised prices for PlayStation Plus more than once, and there's no guarantee that you'll get the same volume of free games or other perks, even if there are good reasons for it, such as hardware obsolescence. Auto-renewal is common and not illegal but they might be an issue.
Andrea Coscelli, Chief Executive of the CMA, said: "Our investigation will look into whether the biggest online gaming companies are being fair with their customers when they automatically renew their contracts, and whether people can easily cancel or get a refund. If the watchdog finds the firms aren’t treating people fairly under consumer protection law, we are fully prepared to take action."
The CMA says that it has written to all three companies, requesting information about such contracts to "help better understand their practices".