I remember the days of placing each step against the handrails and with the third hand applying adhesive!
In 3mm its easy, just buy some Plastruct styrene stair and handrail. It says on the packet HO but then scales it as 1:100- result.
After the stair and rails are cut to length it's just a matter of running some MEK along the joint with a fine paintbrush and it's done. Some end uprights have been added but a few minutes work and we have the steps for Teignfords new Signalbox.