D3, D2, and D1 are just different settings that allow you to control the shifting of the gears to some degree. In D1, the car stays in first gear only. In D2, the car can shift back and forth from first gear to second gear but that's all. So, logically, D3 allows the car to switch at will between first, second, and third gears. For the most part, it's best to leave a car with an automatic transmission in drive because the car has a computer that figures out which gear the car really needs to be in given the conditions (grade of the road, rate of acceleration, etc). However, if you want to go to manual shift, just use the shifter as you would in a stick-shift. Start in D1, shift up to D2 when you want to go into second gear, then D3 for third gear, and back into drive to engage the 4-speed overdrive. Make sense?