The 3rd flatiron is a low-angle classic in the climbing world. It is easily accessible from the Chataqua park trailhead just on the edge of Boulder, is over 1000 feet of sustained climbing, and is a beautifully solid and featured rock with plenty of juggy holds just where you need them. For these reasons, it is very popular and you will almost always have company while climbing. The standard East face route is the most popular, and is typically rated at about 5.4. The rating seems to come mostly from the last pitch, where you are slightly exposed and on a bit more smooth and slabby rock than the rest of the climb. Most of the climb on the standard East fast is class 4-ish or barely stretching into class 5. Most parties still climb it roped, but many climb it unroped as well. I have climbed it both ways a few times, and I have felt completely comfortable at all times. It truly is a classic, and a very enjoyable climb.
Head up towards the flatirons from Chataqua and follow the signs for the 2nd/3rd flatiron climbing access. There are a couple of spots where the trail forks off and leads to scrambles up talus slope, so it's advisable to go with someone who knows the way the first couple of times. The trail will eventually lead to an obvious spot on the North side of the bottom of the 3rd where people begin the technical climb. On decent weather days, there is almost always a party at this spot getting ready to ascend.
For the standard East face route, you will want to find a spot early on to traverse to the left, just past the giant crack that follows the entirety of the East face. Once past the crack, you can ascend directly upwards for the majority of the time, exploring the plentiful holds and features.
When you are about 3/4 of the way up, the main crack will widen and if you are doing the standard East face route, you will want to traverse right early and finish the entire route up the main face just to the right (north side) of the major crack. You could also continue upwards to the left of the crack and then traverse at the top of that slab. See the beta photo for clarification.
The last pitch is excellent, and brings you over the slightly exposed and smoother section of rock to the summit. It is slightly more challenging than the rest of the climb, but is still low angle and very manageable.
Some people downclimb a route on the 3rd, but I think it's best to rappel. There are huge, solid eye-bolts at easily identified rappel stations on the West/Southwest side. With a regular 50m/60m rope you will do 3 rappels. If you have a double-length rope you can do it in 2.
When rappeling, you will want to pay attention to a sign next to one of the anchors. It will point out the fact that the immediately accessible anchor requires a double-length rope to touch the ground, while the anchor a few feet to the West will get you to the ground with a standard length (50m/60m) rope. It's not difficult, but just be looking for it. Apparently many people have died here by rappelling off the end of a single length rope.
After the 3rd rappel, you will scramble to the Northwest and head back down the trail. Route finding here can be tricky as well, so again it's best to go with someone who knows the route the first couple of times. Also look for the trail marker signs and cairns. You will eventually merge with the trail you came up and go back down to the Chataqua trailhead. Total time car-to-car for the standard East face route varies anywhere from 37 minutes (record setting times by world class athletes the likes of Satan's Minions) to over 6 hours (less experienced roped parties).
Have fun! This is a classic!