The only switch to the regular ceiling lamp in my hallway is by the front door. It would be handy to retain this switch function, but to add two alternative switches in the hall so that any one of the three can independently switch the same lamp. Can someone please advise easiest way to achieve this wiring-wise?
An electrician can easily convert what you have now to Two Way and Intermediate.
He will run a 3 core and earth from existing switch to the new middle intermediate switch then a 3 core and earth from there on to the third switch which will be two way .
Pretty straight forward