I took a college class for automatic transmission and the purpose of the manual 3rd was not covered. I know that manual 1st is used for maximum engine braking under 40 mph when descending a hill, and manual 2nd is used for acquiring increased power when attempting to pass another vehicle. But I do not know what the prupose of the manual 3rd gear is because we covered the late model GM 3T40 automatic transmission that only has the manual 1st and 2nd option.