A great route that would be better with more traffic. Presently only a little lichen and a few thin holds (good rock, but thin-edged plates) make the route a little exciting, but even this considered, it is nearly uncompromised fun.
P1 (5.6, 18m): Climb up the left side of the blunt arete and aim for the general vicinity of the bolt. The climbing is perhaps 5.6. Arrange some gear and belay here (as suggested by Rossiter) ...or continue, and have your partner simul-climb to finish the route as you get higher.
P2 (5.8+, 55m): Clip the bolt and pull up and onto the climbing above. Rossiter suggests this to be the crux, but I found it to be 5.8-. Place gear as possible and head up and slightly right, avoiding the easy, (tempting) but dirty and brittle rock ramp going up left and to nowhere...
Continue up and very slightly right to the base of a shallow left-facing left-leaning corner... climb this to the top (crux, 5.8+) with OK gear, minding a few loose "holds" on either side so as not to bomb the belayer). Continue to the top on lower angle rock and mostly positive, solid rock. Belay back from the edge over the top on some hand-sized or larger cams.
To descend, climb down into the S. Chimney, and down climb that (5.3) or climb up to the rock ramp above and then rap from an old eyebolt (2 ropes) or continue up toward the summit of the 3rd and use a standard descent.
Arrive at the Upper South face of the 3rd Flatiron in the vicinity of the S.W. Chimney and S. Chimney. Between those routes, perhaps downhill 30 meters from the SW chimney, and uphill 15 meters from the South Chimney, there is a blunt arete at the base of a gutter of rock leading up. This blunt arete had a 1.5 meter round solution hole to the right and a chute-shaped one up and left, above the base of the gutter and running downhill parallel to it.
Careful inspection of the rock directly above, and on the left of this arete will reveal a silver bolt-hanger on a very small "bulge" about 18 meters up. The bolt appears to be good.
A standard light rack to 4" with plenty of long slings. a 70M rope is useful if doing as one pitch, as it keeps the simul-climbing portion easy!