Ever consider the use of black pipe and pipe flanges?. Most black pipe comes in nominal inch diameters and you can get it in four foot lengths and is usually threaded on both ends. Add the pipe flanges to match the diameter of the pipe and fasten using carriage bolts. Just unscrew the pipe when done using the bench.
Frank

Built a shooting bench from one of those heavy duty, folding leg banquet tables.
Cut down the width and length and added leg extensions for height.
Don't have pictures but could take them if interested.
Would give it to you if you were close as the range now has benches.
Chuck

KW,
Like you, I had to come up with something new, my last one didn't fold and was a pain to haul around.  For a folding one the best I can come up with... I'm still making the hinges, they're not done yet and I needed a bench for a match this coming weekend.  Here is the alternative I came up with Sunday night.  Butcher block piece from Home Depot, 1-1/2" pipe and pipe flanges.  I made threaded inserts for the bottoms and made knurled adjusting knobs to level it.  Fairly stable, not real easy to pack, but a decent cobble job.  I may get the hinges done some day and come up with something better than just the black pipe.   
Once you get your creative juices flowing, please share with us.
Greg

Not three legged, but had friends that used black and decker workmates, with modified tops, for shooting prairie dogs.  They live 900 miles away so I never saw them but from what I understand they added a top to the existing factory top to make it more shooting friendly.  The new top had an added piece that clamped into the workmate, making it easy to remove.
Bob

Ok, seems I will go with a banquet table dual leg set in front, and a spring loaded locking hinge (ebay) for the single rear leg.
Thanks all
kw