A well-graded trail leading from Deer Park to the Three Forks campground.
From Deer Park, head east on a short traverse on the Deer Ridge trail for 0.3 miles. You'll pass through a lovely open meadow with gorgeous views of the interior Olympic Mountains.
Soon arrive at a junction with a trail heading downhill to your right. This is the Three Forks Trail. Turn right and get ready for relentless downhill; the next 4 miles you head down, down down.
The first half mile or so of your descent is on an open ridge, and is the steepest section of your trek. It can be hot, so carry lots of water. Once you get into the trees though, the grade evens out, and it's a pleasantly consistent descent down the mountainside into the valley.
As you near the bottom, the trail begins switchbacking consistently, ultimately delivering you to the well-appointed Three Forks camp. There's a privy and a large open shelter, as well as plenty of spots for tents. Across the river is Gray Wolf Camp. Upstream is Cameron Basin via Cameron Creek, and downstream you can hike out to Slab Camp.
WTA Pro Tip: This trail is dry and there is only one water source at Deer Park, in a campsite. Bring all the water you'll need with you, and a way to filter so you can refill at the bottom for the trip back up.