This issue was apparent before this weekend...
I bumped into this exact issue on the evening of their (last) large outage on the 16th Oct and commented about it in the https://forums.theregister.co.uk/forum/all/2019/10/17/three_uk_down_outage_not_working_borked_parrot_dead_ceased_to_be/ article (comment #75-ish, reproduced below) - despite copying in the ICO, providing an image of someone else's bill and chasing numerous Three numerous times via Twitter, I've have absolutely zero response... Good to see they're taking responsibility...
"In other TITSUP news...
The network issues are annoying granted, but possibly not as important as being directly logged into someone else account when hitting the My3 URL last night at about 19.50 and being able to browse all their call logs/billing details.
Appears remarkably reminiscent of: https://www.theregister.co.uk/2017/03/21/three_admits_to_data_breach/
So far, no response as of yet on Twitter from Three Support on that one..."