1 - 4 WAY SWITCH+ 2 - 3 WAY SWITCHES (Total of 3 switches controlling same fixture), Power at 2nd Switch, Fixture feed from 2nd Switch
At the ceiling box - the ungrounded conductor {aka hot} from the fixture (could be a brass colored screw) connects to the black wire from the switch 2. The white wire from switch 2 is the grounded leg conductor {aka neutral} for the fixture (could be a silver color screw).
Wrap a piece of black electrical tape around both ends of all white wire going between switch box 1 and 2, and between switch box 2 and 3 as these wires are being used as a ungrounded conductor {aka hot}.
Bare wires are together and grounded to box (if metallic) and if the fixture has a green wire or a grounding screw , it gets grounded also. Bares and green wires are equipment grounding conductors.
Be sure to read any documentation that comes with your fixture
If only switched power required onward then at existing fixture...
If only unswitched power required onward then at existing fixture...
Onward unswitched power is not possible with this configuration as no unswitched ungrounded conductor {aka hot} exists at the fixture location.
NOTE: Onward power is only available at switch box 2. Onward power is not possible from switch box1 or 3 as no grounded leg {aka neutral} exists at switch box 1 or 3.
If only switched power required onward then at existing switch 2...If only unswitched power required onward then at existing switch 2...
If switched and unswitched power required onward then at existing switch 2...
The common screw on a 3 way switch is the odd color screw
If wiring multiple fixtures be sure to check the following guidelines concerning how to do it and what not to do: Wiring Multiple Fixtures.
If you are new at understanding the "politically correct" electrical terminology and need clarification of the terms used, we strongly urge you to read the Clarification of Definitions and Terminology guide. This will explain the terminology used in this article.
Also, be sure to read "Identifying the Correct Screws on 4-Way and 3-Way Switches".