The trailer weight is how much it weighs. Not how much it weighs minus the pin. Your truck still has to tote that 3,000 pounds too. It doesn't magically go away when it presses on the rear axle.
The truck in question would not be overload as you're not exceeding the GVWR. However, you would be towing over the advertised limit of the truck because the whole rig weighs 26,000 but the GCWR is only 23,500.
Tow rating is not a legally enforceable number. Other than going slower up hills I wouldn't have a problem towing that much with that truck.