Dryers are 220 Volts AC and the outer connections inside the dryer are "hot" or live legs (black or black and red wires) of the circuit, while the center one is typically the neutral (white) wire.
The difference between a 3 prong plug and receptacle and a 4 pronged one is simply that the 4 prong plug and receptacle has a dedicated ground integrated into it, whereas the 3 prong plug is what's commonly referred to as an ungrounded circuit. There is no dedicated ground in a 3 prong plug and receptacle that makes a separate, grounded connection from the dryer to the receptacle to the electrical panel of the house.
Also, please look at either the back of the dryer for a basic wiring diagram, or look inside the access panel where the dryer cord is connected. There should be indications as to what each terminal is for, as I stated, typically the outer connections are the "hot" while the center one is typically the neutral. If you notice a strap running from one of the dryer's connection terminals to the cabinet/frame of the dryer, that is the neutral. The others are the hot terminals, which are the outside ones.
Look for a green grounding screw inside the dryer's electrical connection section. That is where the green wire on the cordset would be connected/terminated.
If you wish, you can get a free Installation Guide for your dryer at the following link to download and/or print out at:
It will show you the connections for both a 3 and 4 wire cordset.
Hope you find this Very Helpful and best regards!