My take on the OP. Your meter is the original meter and the only one registered with a supplier. The building has been split up into flats and rather than getting new supplies / splitting the original, they have just installed 2 additional private pre-pay meters (sub metering). The original intention of this is that the main supply tenant would supply the pre-pay cards to the sub metered flat tenants and then the money from them basically pays their part of the bill. This used to be very common, and probably still is to some extent.
You are now at the very bad end of a crap stick. You are essentially paying for all 3 flats. The sub meter on the left have been by passed so never runs up. No idea who is suppying the other tenant with cards, but they are pretty readily available, cheaply. So probably just buying their own.
Now the flats have all been sold on without keeping the agreement in place. It never was a good idea, this setup.
You can try ask the DNO / supplier about adding additional meters, but they will more than likely want to upgrade the upply, which could cost big bucks.
Are the other 2 owners approachable, to explain the issue to and try resolve?