As noted by ericnoel Three Way Peak is named for being on a triple divide between the Crystal Creek, Morse Creek and Elizabeth Creek Drainages. Also it is the 3 way junction of the Mount Rainier National Park to the West, Mount Baker-Snoqualmie National Forrest to the North and Wenatchee National Forrest to the South East.
From Seattle head down I-5, west on 18, make a left(south) and stay on that road until you reach Enumclaw, make a left(east) onto 410, make a left onto Crystal Mt. Blvd. Parking is free.
Three Way Peak is just South of Crystal Mountain Ski Resort. Roughly a miles hike following the South ridge from the top of The Queen(7002') which is accessible via the High Campbell chairlift(Chair 6). You will pass the Throne, Silver King and the Box Car Cornice before reaching Three Way Peak.
It is uncontrolled terrain as noted by this sign.
Once you reach that sign, traverse around the West face.
There will be a steep chute just after the biggest rock face along the traverse, head up from there.
Next traverse around the South face just below the next big rock face then on up to the ridge.
From there the summit is just 100' or so west along the ridge. I would suggest being on belay if you want to reach the true summit.
I'm not sure what the official stance is on climbing/riding this peak because the typical route requires you to cross all 3 boundaries to the various forest lands.
Again depending on which side you camp on you may have issues, but because it's so obscure a location not many people get back there. I have seen one person camping on the north side(Snoqualmie-Baker)...but that is not to say it is the "correct" side to camp on.
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I'm not sure how many people visit this peak, but I am curious to find out who else has climbed it.