I remember reading about a guy who had added a 3rd axle to his 5th wheel, so it can be done. It looked pretty normal when he was done.
I would think the axle behind the other two would be the way to go. That would help support the weight of the load behind the axles. It would be good to know what you meant when you said "I cut the first 8' off the front last year and rebuilt everything". Were you repairing the unit or remodeling?
And yeah, you really need to get it weighed so you know what the numbers are.