Total of 3 switches controlling same fixture), Power at 2nd Switch, Fixture feed from 3rd 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 3. The white wire from switch 3 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 as this wire is being used as a ungrounded conductor {aka hot}.
Bare wires are together and grounded tobox (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...
(2) 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: Only switched power is available at switch box 3, and only unswitched power is available at switch box 2. Onward power is not possible from switch box 1 as no grounded leg {aka neutral} exists at switch box 1.
If only switched power required onward then at existing switch 3...
If only 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".