I keep seeing references to a three way switch. Where is it located and what does it do. I have a 3389B trailer.

Near the entrance door ,down low, next to the raise and lower switch and release cable.

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Les & Patricia
2006 22ft towlite (replaces 97 21TL)
2006 Toyota tacoma pre-runner 4.0 L V-6 quad cab
w/ 98 chevy Powervision towing mirrors
2017 Chevy Silverado 1500 5.3 v8 (replaces Toyota)
2 Hobie mirage pedal drive kayaks

Your HiLo may not have it, they changed the switch arrangement somewhere along the way. Someone may come along and fill you in on that.

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My Great Wife Joyce
2018 Jayco Jay Flight 21QB
Formerly owned 1705T and 2310H
2012 F150 4X4 SuperCrew EcoBoost w/Leer Cap
Reese WD Strait-Line Hitch
Amateur Radio K3EXU

the only thing by the up and down switch is the down release cable and a red light.

To actually answer your question, now that I have read it again, the three way rocker switch controls power to either the lift circuit or the 12 volt systems to the rest of the trailer. Up position to 12 volts, middle position all off, and bottom position to lift system.

__________________
My Great Wife Joyce
2018 Jayco Jay Flight 21QB
Formerly owned 1705T and 2310H
2012 F150 4X4 SuperCrew EcoBoost w/Leer Cap
Reese WD Strait-Line Hitch
Amateur Radio K3EXU