According to SMACNA -duct leakage test manual, It is not recommended that duct system constructed to 3" (750Pa) wg class or lower be tested. My question is "what is 3" (750Pa) wg class?"

A Pascal is a metric pressure measurement. Based on your post, 3" water guage is equal to 750 pascals ( 1" is 250 Pa.) So any ductwork that is not constructed to these minimums 3" WG(water gauge) or 750 Pascals should not be tested. I would guess so as to not damage the duct work.