I've done it, and I enjoyed myself, but absolutely hated the exit and would not go again without an improvement in my skills.
Three canyon is south of Green River, not far from Moonshine Wash. It runs basically parallel to the gravel road, a mile or so over to the east. Because of the proximity of the road, dropping off a shuttle vehicle at the exit saves a lot of walking time.
Another good reason to drop a car at the exit, is that the canyon exit itself is quite steep and I consider it risky for beginners. Luckily there are two existing bolts, supposedly placed by a farmer so he could raise his sheep up the canyon exit. The move is to have an experienced canyoneer (Mike in our case) head down the exit with two handlines. Attach a handline to each Sheep Bolt, one 1/3 of the way down, and the other 2/3 of the way down.
(If you have an AJRoadTrip subscription, he has pix of the bolts, and of someone being belayed out of the canyon in a harness.)
Enough beginner warnings - On to the canyon:
AJ's beta is not exactly correct - it suggests there's two 50 raps. In reality, there's one 90' rap, which is bolted and straightforward. Next to that rap, there is another alternate rap, which is shorter, which dumps you onto a ledge that you would then have to rap. I guess that's where the two 50' raps comes from, but I'm not sure. I can tell you there's not two separate raps during your trip through the canyon, even for a beginner/intermediate...
Looking at the first and only Rap
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The canyon itself is pretty. The rap is fun - there's even a place to hang out and watch the rap from another ledge. Unfortunately the exit is several miles after the last rap, through some mud and a lot of vegetation when we did it Oct '12.
Once you locate the exit canyon, you'll have to walk along a narrow shelf about 30' above the canyon floor. Once you've done that, you're looking at a 300' climb out. It's steep and I found it scary but I have a fear of heights. Without the handlines we dropped, there's absolutely no way I would have gone out that way. About 1/2 way up the climb, you have to negotiate over a cave-like hole in the rock face; a fall there would mean you'd likely be rolling and tumbling all the way back to the canyon floor.
Looking across the exit, watching our experienced Canyoneer Mike place the handlines.
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I wish I could say I was bad mouthing it so it wouldn't get too crowded - but honestly that's not the case. It's a long drive for one rap, all the vegetation, and the lousy exit.