3-pole means that the device plug is earthed and that it normally has three pins, of which two are used to transfer the current and one is used as personal protection.

Due to the predominance of more powerful devices in the 1930s (normally above around 500 watts) the problem of a possible electric shock became a hazard for the user. In order to provide safety, an earth connection had to be fitted to the 2-pole plugs for these devices. In all plug types, the earth connection was fitted as a 3^rd pin. The exception is the Schuko plug in which the earth connection is located on the side of the plug and made of metal contact surfaces.

Removal of the earth connection is life-threatening in any case since there would no longer be protection against electric shock! In the case of all 3-pole plugs which conform to the standards, this earth connection has been constructed as a leading connection, which means that the earth connection closes the contact before the other two pins make power contact.

You can find a device overview here. 

In most countries in the world, the 2-pole mains sockets have been replaced with 3-pole earthed sockets which can accept both 2-pole and 3-pole plugs.