Each U.S. Army battalion and brigade has a military staff. This staff is comprised of officers who are responsible for a unique functional area (intelligence, supply, personnel administration, etc.) that is necessary for the unit to complete its mission. These officers answer directly to the executive and commanding officers and are designated by the code "S" and a number relating to a functional area. The S3 officer is in charge of operational planning and training at the battalion and brigade level.
When the unit is not deployed, the S3 is responsible for training. The S3 creates a list of tasks that both individuals and the unit should be able to perform. They use this list when grading unit performances during exercises.
The S3 is also responsible for planning operations when the unit is deployed. They write the standard operational procedures (SOP) for situations that may come up during combat. They also generate the orders for specific combat actions such as directing a subordinate company to secure a hill. Finally, S3s evaluate performance in the form of After Action Reports.
S3s start their careers in much the same manner as other staff officers. They go through the initial training to earn their commissions through Officer Candidate School, the Reserve Officer Training Corps or the United States Military Academy. After becoming second lieutenants, these new officers begin their branch training (infantry, intelligence, aviation, etc.). After a few years as platoon leaders, they are promoted to captain and attend the Captains Career Course (C3) for their branch. In the C3, captains learn how to lead troops on the company level and how to perform the jobs of staff officers. While officers are generally assigned to staff positions related to their branch, it is not unheard of for cavalry officers to serve as S3 in an infantry brigade.
S3 officers have a tremendous influence on the effectiveness of battalions and brigades. Therefore, they must have excellent communications skills because they determine how the unit will respond to given conditions. Every one from the commanding officer to the lowest private must understand the S3's plans. Also, S3s must have strong interpersonal skills since they will have to work with all the other staff officers in developing those plans.
The average infantry division has five brigades and five battalions for each of those brigades. Currently there are about 20 divisions in operation. Combined with the battalions and brigades from non-infantry divisions, there are many assignment opportunities for officers aspiring to an S3 position. Most of these positions are in the U.S., but there are also many more in Europe and the South Pacific.
After the Army
There are many opportunities for Army S3s who with to transition to the civilian world. They can go on to work as operations mangers or vice president of operations for corporations. S3s who work in technical branches such as Signals of Military Intelligence might also find work as researchers.