Class 3 material is a designation of the Resistannce Welding Manufacturers Alliance (RWMA). It describes a group of copper alloys with excellent strength and good electrical conductibity. Class 3 is often used to weld stainless steel, nickel alloys and other highly resistive - strong materials that require high weld forces.
Class 3 Bar Stock
Because of it's excellent strength welding gun arms are frequently manufactured using Class 3 material. It is used for projection welding and back up electrode applications due to its high hardness and strength.
The product description and properties of Class 3 can be found in the Resistance Welding Manual Section 18-3, available at RWMA/AWS.
There are other Class materials with other properties all shown in the above reference. Their properties allow for welding work pieces with both higher and lower resistances and strengths than stainless and nickel alloys.