4X8 trailer and 3 full size bikes

The widest part of the bikes are the handlebars.

If you can arrange your trailer so that the center bike sits farther forward, up towards the tongue, then things will fit better.

If you can't do that then plan on having the two outside bikes facing forward and the center bike facing backwards.

It is going to be a tight fit and you will want the bikes as wide on the trailer as you can manage. What you might want to consider is throwing down some angle iron or square tubing across the trailer to provide an anchor point 6 inches or so wider than the trailer. You can certainly go as wide as the wheels/fenders without any issues, and even farther if you put reflectors and yellow lights on your outriggers.

A railing all around and some planks to make a box is a nice luxury but I doubt that you will have room for it. Perhaps if you leaned the railing out so that they gave you enough room for the foot pegs it would be OK.

Channel guides are highly recommended but they don't need to be fancy. I used to use an old Jet Ski trailer for hauling my bikes. I simply slapped down a 4'x8' sheet of 3/4" plywood, used the outriggers I mentioned above, then put down 12" long strips of the angle iron right where the tires would ride to keep the bike from sliding sideways.

Rod

I've put 3 bikes on a 4x8 but it was one full sized, one KX85 and one PW80. The two bigger bikes face forward and the PW is in the middle facing the rear. The only rail I use is one made of 2x4s on the front and back. It's tight but surely do-able.

Might try the two minis side by side next time.

i did the same 2 big bikes and a 50 in the middle.