...If it is less than 3 feet long, you can use the 20 ft. trailer...
Shouldnt that be, if it is at least 3 feet long?
If it is less than 3 feet from the rear axle to the back end of the truck, that means it's more than 20 feet from the rear axle to the front of the truck, and the rear wheels are also going to be hanging off the back of the trailer...