The ground blade on a three blade plug (cap) is to provide a low impedance return to the voltage source. If the portable equipment develops a short in its circuit and goes to ground, it is this blade that will carry the short circuit current back to the supply distribution panel and trip the breaker open to cut off the supply voltage.
Need to know the amperage rating of the plug or the NEMA configuration of the plug and receptacle
Yes, provided the outlet has a three element receptacle. The third prong is ground. If you use an adapter that connects three prongs to a two slot outlet there is a shock danger.
The 3rd round plug is the ground connection.
Buy a new receptacle that your plug fits in. You probably need one with a neutral so look for 125/240v.
An old 2 hole receptacle can be changed to a 3 hole receptacle that will accept a 3 prong plug, provided a ground wire is available at the box and connected to the ground (green) lug on the new receptacle. A 2 hole receptacle has a hot and neutral wire, while a 3 hole receptacle will require a ground wire connection -- in addition to the hot and neutral wires.
On a three prong plug there will be a neutral blade, a "hot" blade and a ground blade. The neutral blade is wider that the "hot" blade. In North America the ground blade is "U" shaped.
Ground wire
the ground plug should be up
Look in the back of the range receptacle box that is in the wall. The three wires coming in should have a bare ground wire in the cable set. It wasn't brought to the receptacle because there was no place for it on the three wire receptacle If you find it back there under a screw terminal just add another short piece of wire under the screw and then connect the other end of the short wire to the new ground terminal on the new four position receptacle The wire should be equal in size to the size of the wire that exists around the ground terminal now. If the house is so old that the range cable did not have a ground wire in it the code allows a separate green ground #10 wire to be taken from the breaker panel box to the existing range receptacle This wire is to be bonded on each end. At the panel end to the ground buss and at the receptacle end around the ground screw at the back of the box unbroken and then to the new four position receptacle ground lug.
Under the new electrical code rules three prong dryer cord receptacles and therefore cords are not allowed to be installed. As you are finding out, stores are not carrying them in stock any more. You should consider an upgrade for safety sakes, to a four prong plug cord and change the wall receptacle to a four prong receptacle. There are instructions on how to do this your self on Answers.
My question is WHY did you replace a four prong dryer plug with a three prong! 220 volt Electric dryers require two hot legs, a neutral and a system ground wire. Sounds as though you shunted one of the hots or the neutral. You need to install a four prong plug of the same configuration and wire it exactly as the original.
The three prong plug incorporates a ground wire for safety. If there is a short circuit in an appliance that has a three prong plug, the current will travel back to the electrical panel and either blow the fuse to that circuit, or trip the circuit breaker to shut power down on that circuit.