When starting a combat round, you start with 3 U.S. markersin short range behind your tank in sectors 1,2, and 3
As I understand it this is not fully correct. At every moment there can be only one US Control marker per sector. This is valid for all ranges in a given sector. So no reinforcements when there is a Control marker. See paragraph 5.12 for reference. In the rules there is no mention of placing Control markers into a particular range of a sector.