Kearney's rank as a brigadier dates from May 17, 1861
Sickles' rank as a brigadier dates from September 3, 1861
Stoneman's rank as a brigadier dates from August 13, 1861
Sickles was on leave for 2nd Bull Run and Chantilly. He was at Washington with III Corps during Antietam.
Stoneman was in the cavalry.
Kearney died at Chantilly.
Kearney, senior in rank to both men, most likely would have been appointed to command of III Corps when Heintzelman left in October of 1862. Assuming he had survived Chantilly, of course.