Plumbing in a toilet in block of flats

are you shore it is 3 inch ?, is it iron lead or plastic ?,if it is 3 inch it is a waste stack for bath basin and sink wastes and not for sewerage even in 1890s soil stacks were 4 inch lead or cast iron,as a waste stack it may discharge into sealed gully at its base,with sewerage this would block forcing sewerage up through wastes such as showers etc,

if it's definitley a 3 inch stack this could be causing the issues with all toilets connecting into this 3 inch is an old cast pipe normally that goes up to vent and potentially picking up a basin and bath definitley not a toilet!

all toilet stacks should be at least 4 inch, 3 inch should vent at top of building with possibly a gutter connecting with water waste from shower, basin ect. I would contact person or landlord from top flat and ask to have this rectified.

I would start with clearing the low level blockage and then survey the pipework to ascertain if a mis-connection has been made. try dye testing from the upstairs toilet you mentioned and see where it discharges.

you find in most old buildings of that age the pipework is cast iron and sometimes they run 2 pipe system, the first is a 4" for toilets and venting and then a smaller 3" for sinks and baths these pipes scale up over years, and the internal bore of the pipe reduces, the pipe goes from 3" to 2" sometimes even less, so as you can see if the contractor put the toilet onto the 3" system even owe 3" would be capable of dealing with a toilet being put on it (not with regs nowadays) over the years the pipe would have scaled up to a point where it could not cope with the toilets waist, you could try cleaning the 3" pipework with an electrical rotating spring but this is very labour intensive and will need a CCTV prior and after, I hope this helps.

It would defo be a blockage then that is causing to push up through basins and shower etc on ground floor, soil pipe should be 4” at least and only 3” for a vent and the fact that it’s picking up more than 1 toilet definitely make me point at it being a blockage in the pipework

Most stack pipes. Svp should be 4 inch 100mm in diameter . If there smaller then blockages will more then likely occur . Best thing to do is get a company in with a camera to send up through the man hole to see initial problem . It could need jetting first to clear blockage . Then remedial advice giving , to increase size of stack to 4 inch , as if it’s old could be cast iron , and over time they corrode and pit internally . Then once this is sorted , to make sure toilet pipe has adequate fall so waste can run away.