This is a fine hike that many should try at some point - it's 4.4 RT miles and 800 feet of gain for the 'short' version described here, with a summit pinnacle that makes for a fun scramble if you feel the need to get to the tippy top. 

From Portland, you'll take I-84 east into the Gorge and exit at Cascade Locks. Take Bridge of the Gods to cross the river and turn right (east) on SR 14 to milepost 43 and turn left on Rock Creek Drive (Skamania Lodge). At 0.3 mile, just past the entrance drive for Skamania Lodge, turn left on Foster Creek Road which shortly becomes Ryan Allen Road. At 0.9 mile, turn left on Red Bluff Road for 0.3 mile, then continue on gravel DNR Road CG 2000. Take this winding, uphill road along Rock Creek. At about 8.5 miles, the PCT crosses this road and you can start the hike here if you like. But for a shorter hike, continue going uphill on CG 2000 to where it tops out at Rock Creek Pass where CG 2000 curves to the right, but go straight ahead and up on Road CG 2090 for 0.3 mile to the Pacific Crest Trail (PCT) crossing. There's a wide spot here and parking for about three cars - sometimes you'll see a bit of flagging tape. Trailhead is near 45.7493° -122.0432°. More directions here.

Take the PCT south (right hand side) and head up, eventually through several switchbacks about 1.5 miles to a right turn. The sign pictured below is large and obvious, but it is sometimes knocked down. Take this Three Corner Rock trail about 0.5 mile to a rough road where you will go right again and very shortly come up to the summit. 

The photo below was found online - we were not lucky for weather or visibility. There is a fun operating position from the very top for VHF/UHF, but it may be easier to set up antennas and operate from lower down on HF. There are a few old fenceposts that can secure a pole for an antenna, but as you can see, no trees on this summit.

 Fire lookout history: https://www.firelookout.com/wa/threecornerrock.html