What is 3" (750Pa) wg class?
Pa (Pascal) is a unit of pressure (air or hydraulic pressure).  It's a very tiny unit, usually used in the thousands or millions.  

"inch" in this context is "inches of water", another unit of pressure.  This one's easy. 

Get a glass of water and a clear straw.  Suck on the straw just enough that the water comes up 3".  Or blow hard enough it goes down 3".  You guessed it - that is 3" of pressure (or vacuum).  AKA 750 Pascals.  You can do the same by plumbing a clear plastic hose into your duct and dipping it in a glass of water. 

Since that's a pretty small amount of pressure, so I can guess this relates to the strength of a blower in a furnace's air handling system. 

Just so you know... you've met "inches of water"... there's another one, "inches of mercury" often used by meterologists.  Same concept, but mercury is 14 times heavier than water.  Also "feet of seawater" for divers, and lots lots more. Clearly, the standards committee was bored.