Get 2 yards at a time on your trailer and 1 in the truck. You can't get the trailer to the back but you could back it to the gate and use the dingo to scoop it off the floor of the trailer. Then drop the trailer and take the truck into the back and dump the 3 yard into the new bed. Now you only made 2 trips.
Knowledge is knowing a tomato is a fruit, wisdom is not using it in a fruit salad.