First, figure out how much you want to take off. Let's say it's 3". I can think of a couple ways to do this. Both ways require a perfectly flat floor - so you should actually double-check that with a long level first, to be sure there isn't any major sagging or something.
One way is to use a level and measuring tape, and place a mark exactly 3" up, plumb from the ground. Using the level avoids the curve in the legs, and will ensure that you're precisely 3" up from the ground.
Another way is to use a laser level that can project a wide, flat beam. Place it 3" from the ground, and then project the laser onto all the legs at once, and then mark everything with a pencil.
It's probably not a bad idea to use both methods, or at least measure twice (there's a reason the saying: measure twice, cut once exists). Both methods require very accurate marks, so be sure to use a sharp pencil and always mark at the exact same spot (like the top of the laser line). Place 2-4 marks around the edge as need. You can now use a string or wire, and draw a line all the way around, or use a piece of tape, to perfectly mark the perimeter of where you need to cut in order to get a flat, level cut.
Once you have marked everything, you need to cut the legs. This can be tricky, since you also need to get a flat cut. Without seeing the actual legs, it's hard to suggest exactly how to do this. The legs may come off the table, which will make things easier.
I'm thinking that using a miter saw with a jig or fence set up 3" away would be a good method, so long as you can physically get the legs in there. Place the bottom of the existing legs flat on the jig, and then double-check the blade is going to come down exactly on the line you made earlier. Be sure to account for the kerf.
You may also be able to get a hand miter box around the leg, clamp it down once it the straight cut is lined up with the leg, and then use a hand saw to make the cut.
If you're really good (careful and accurate) with power tools, you may also be able to use a jigsaw or an oscillating cutter. I'd avoid circular and reciprocating saws, they're not very accurate for this type of work, although if you're really good, a circular saw may be able to do it.