Pectin manufacturers know their products well.  Each brand of pectin may vary slightly in quantity used per batch, depending on the ingredients and fillers, but should have the same gelling strength.  We always suggest that consumers follow the instructions with the specific pectin they purchase. I see that on the packages we have here, they also say 3 T is the equivalent of one box. In our classes we switch back and forth between individual boxes and 3 T.  bulk pectin and have had success with both products. 

All pectin manufacturers have customer service numbers or emails which should be on the label.  Don’t hesitate to contact the company directly if you have specific questions about their product.