Dryer Cord ground prong removal
The 4-wire connection includes hot, hot, neutral, ground. The 3-wire connection includes hot, hot, neutral, but no ground. The basic premise is that older houses only had the 3-wire receptacles and since ground is essentially the same as neutral, and dryers are rarely moved/replaced, let the 3-wire connections continue to be used. However, when something goes wrong, having ground the same as neutral can be deadly.

Almost every dryer built in the past 30 years or so can be wired up 3-wire or 4-wire. If it is 3-wire, there is a jumper (short piece of wire and/or a screw in the right place) to connect ground and neutral. If it is switched to 4-wire, that piece of wire or screw is removed.

NEVER mess around with the actual plug. Either use a 3-wire plug with a 3-wire receptacle or, preferably, use a 4-wire plug with a 4-wire receptacle.

It is possible to replace a 3-wire receptacle with a safer 4-wire receptacle. However, the amount of work involved varies quite a bit depending on the particular situation:


Dryer receptacle near main panel - easy
Ground wire available, just not connected - easy
Metal conduit from receptacle to main panel - easy
None of the above - hard - need to run a new cable (all the wires) and/or a ground wire, or install GFCI protection instead of ground


There are a bunch of questions on DIY about this, such as How to tell if Nema outlets have been successfully brought up to code?