If my tri axle dump truck has a GVW of 70000 can weigh up to that much what if I add a 20000 lb trailer what if I add a 25000 lb machine?

GVW means Gross Vehicle Weight. That's the maximum permissible weight of the truck plus the trailer (in the case of a combination rig) plus the load. Therefore...if you take your dump truck, hook a 20,000-lb empty trailer to it and put a 25,000-lb backhoe on the trailer, the truck needs to weigh less than 25,000 lbs if you want to remain legal.