The Standard East Face Route up the Third Flatiron is an ubber classic and super-popular climb, especially for beginners and first time leaders. Consequently you'll probably never be alone on the face. Although it is considered a slab climb, the surface of the rock is wavy and textured to provide abundant, quality holds wherever you look.
I shot a short movie when I climbed it with brenta and my wife. You can find it here. Also check out a trip report by CharlesD and the abundant info on climbingboulder.com.
Like most of the Flatirons, the Third Flatiron is closed for raptor nesting February 1 through July 31.
From Chautauqua Park hike up Kinnikinic Road to Bluebell Shelter and find the Royal Arch Trail. Hike south down the Royal Arch Trail for about 3/4 of a mile until you encounter the the Second and Third Flatiron approach trail. Take a right here and follow this trail to a T. Take a left on the Third Flatiron Trail and follow it to the East Bench at the bottom of the East Face.
Pitch 1: Starts from the obvious belay ledge on the East Bench and angles left across the channel to an eyebolt.
Pitches 2-5: Choose the path of least resistance up the face. Eyebolts can be found at the tops of most of the pitches if you have a keen eye (they tend to be over to the right near the channel). If you can't find them it's no big deal because there are plenty of spots to set up a belay. Pitch 5 crosses the "C" of the painted "CU".
Pitch 6: You can either cross the channel here or continue up to the left of it. (The rest of the description assumes you stayed left). Work your way up onto the top of large bulge. If you can make your way to the top of the bulge then you can make it to the summit in one final pitch.
Pitch 7 (or 8 if it took you an extra pitch to make it to the top of the bulge): From here you can traverse across the gash onto Kiddy Car Ledge. The next 60 feet provide the most interesting climbing of the day. Here the holds get smaller and farther apart and it feels a lot more slabby. Several sources say the final pitch is runout but I thought this was over-hyped.
From the top you can make three rappels or one long, free-hanging rappel to get you to the saddle west of the Third Flatiron. The single, long rappel can be achieved only if you have two 60m ropes AND somebody at the top to untie and toss down your ropes to you once you're down. There is too much ropedrag for you to be able to pull them down from below.
Thus the three rappels are considered the standard descent. Find the first eyebolt on the south side of the summit. Make a short rappel down to the second eyebolt. From the second bolt make another short rappel down to the Friday's Folly Ledge. (mtnsavy notes that with a 60m rope you can combine the first two rappels so that you arrive at Friday's Folly Ledge in a single rappel. This can save some time especially when there is a crowd waiting to get down.) From here you will have to traverse about 15 feet to climbers left to find the third eyebolt. Make the final, longer rappel to the ground.
Once you've made it to the ground west of the Third Flatiron hike north. Descend until you find the cairned climbers trail that will take you around the north side of the Third Flatiron. The climbers trail will eventually connect you to the East Face Trail where it crosses a boulder field.
Bring a standard Flatirons rack. I think I used a couple of small stoppers and cams from a yellow Alien through a #3 Camalot. There are a few horns and chickenheads to sling so a couple of extra runners could be useful.
A 60m rope will get you up and down via three rappels. If you bring two 60m ropes you can make one long, free-hanging rappel off the summit. However, if you choose this option you will have to ask another party on the top to untie your ropes and toss them to you when you're down. Rope drag is too significant for you to pull them from below.