im currently adding a 3 way switch in the upstairs hallway. There is already a switch at the top of the stairs and one at the bottom which work 2 way with each other for the landing lights. The drawings below are the wiring of the current switches.
The one on the left is the downstairs which has one 3 core cable in and the one on the right is the upstairs which had x4 cables which are x3 red singles and the 3 core from the other switch. Both switches are 1 gang with a c, l1 and l2. The switch i am adding is 1 gang with a c, l1 and l2 and i have ran a 3 core from the upstairs one to feed it. I have also brought an intermidiate switch to replace the right hand switch in the picture. Can you tell me how to wire the new switch and the new intermidiate switch which will be replacing the switch on the right. Thankyou so much for your kind help.