I had a 16 ft 7k utility trailer that I made ramps for and converted it into an implement trailer.
Last year I bought a new 20 ft 14 k implement trailer with a 2 ft dovetail and 5 ft ramps.
If I can get the pics to work, I'll show you how not to build ramps. You want the knees to be in front of the hinge pin not behind the pin, like my new trailer can with. The ramps would kick up on you when loading or unloading a heavy object.
As for using 3 inch channel, I think it will work. Here is why. I load a 4640 pound lawn roller on my ladder ramps which are made using 3 inch channe. Think about it. A 4600 pound tractor would not have all it's weight on the ramp at once like my roller does.
If I were you I would build 5 or 6 ft ladder ramps and have them hinge on the end of the trailer and make then slide for different size wheel bases. I wouldn't put a dove tail on the trailer, because another 4 ft will give you a tailgate swing. You turn and the tail of the trailer may swing into something.
Instead of a dovetail, I would make shorter knees on the ramps and make sure the knees are in front of the hinge. I'll show you a pick of how I did that. Start out by putting perhaps 1 ft of blocking at the end of the trailer. This will prevent the tractor from putting the end of the trailer on the ground. After you have the ramps attached to the end of the trailer, take your heavest tractor you want to transport and put the front wheel almost on the trailer. You will find the springs of the trailer will convert your 20 ft trailer into a large dove tail. If you like the incline angle, measure the distance from the ramps to the ground and make your knees that long. If you like the distance the trailer is compressed, go ahead and pull the tractor all the way on. Make sure your knees are short enough to lift the ramps. This is what I did with my new trailer. I like the ramps much better than the factor ramps. I like the shorter knees, my incline angle is much less too. Lift springs are nice too.
LOL. Post back and let us know how it works.