Hello I will help you with your question,
Drive 1 is first or lowest gear ratio and should be used for the slowest speeds when you do not want the transmission to shift up in gears.
Drive 2 can be used if you are in slipery conditions and you are driving slowly and do not want the added torque of drive 1 to break the traction of the drive wheels.
Drive 3 would be used in a towing situation when you do not want the transmission to upshift to overdirve where it may be under excessive stress.
Drive 4 is for everyday driving and allows the computer to control the trasnsmission shifting depending on what its input information reads.
If you have specific questions about this please let me know
Yes you can drive in 3rd all the time but your fuel mileage will suffer as 4th is an overdrive gear and allows the engine to turn slower at road speeds. If all you do is around town driving 3rd will be fine but 4th is preferable.