You'll probably have at least half of that 3K into modifying your existing trailer. If you're going to do that you might as well just build a new one. the problem that you'll have is that most dual axle trailers link the axles via the leaf springs and you're just looking to add an axle. You'll have the devil's own time getting it positioned right, you'll have to move the original axle, to carry the load evenly on each axle.
Now, if you want triple axles cheaply, just find somebody with a new mobile home they've parked permanently. And, you might even keep your eyes peeled for a junk mobile home-then you've got all the steel as well to make a pretty hefty trailer, cheap. You'll save on the tires alone! Look for the trailer park where known meth-amphetemine cooking is taking place; when they screw up they tend to burn off all the spare parts you don't need. (g)
Otherwise, you can probably beef up the trailer you've got. Stiffeners can easily be welded on as well as additional leaf springs. It really doesn't take much to beef it up for another 500lbs capacity.
All these ideas presuppose that either you or a really good friend knows his way around a cutting torch and an arc welder.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.