You'll set up 3 scenes:
Scene 1 the top button will be the trigger and you set the actions to turn all on
Scene 2 the middle button will be the trigger and you set the actions to turn all off
Scene 3 is where it gets more interesting.... make a scene with the bottom button as the trigger and dont set anything in device actions. get yourself to step 3 of the scene creation where you can see the section "Also, execute the following Luup code:" you will add the following but you MUST change the dID from 23 to whatever the device ID of your stairway light is. This code will check the status of the light...if it is on it will turn it off, if it is off it will turn it on. You can find your device ID by clicking on the device in the dashboard and then hitting advanced. At the top of the advanced page in larger font it will say "device #__".
local dID = 23
local status = luup.variable_get("urn:upnp-org:serviceId:SwitchPower1","Status",dID)
if status == "1" then
luup.call_action("urn:upnp-org:serviceId:SwitchPower1","SetTarget",{newTargetValue=0},dID)
else
luup.call_action("urn:upnp-org:serviceId:SwitchPower1","SetTarget",{newTargetValue=1},dID)
end