Granite countertop installation on top of 4x4 outdoor
1). The answer is really time. Nothing lasts forever. The more activity it sees the shorter the lifespan will be. Obviously, supporting the edges would make it more robust. 

Your design may last many years, but I would be tempted to use angle brackets or cross members on the ends to prevent wobble, or perhaps use four legs (one in each corner) but that may be a matter of function vs. style. Otherwise, there are a vast array of angle brackets which would be suitable for this. Some factors to consider are whether or not you want the brackets to be ornate or hidden; material considerations (wood, wrought iron or even cable possibly); and of course, budget. 

2). There may be regulations (safety tests) for commercial chairs (depending on what the retailer wants)... or if you build this for someone else, as part of a contract, you might need to stipulate repairs and liability in the contract. 

As for three-leg-placement options, I think that the golden ratio is generally good. As such, I would center the outer posts at 2.472 feet (approximately 2', 5-11/16") on either side of the center post. Of course other arrangements will also work.