Why do space suits have to be pressurized to three atmospheres?

Perhaps for testing purposes, to check for leaks. There would be no reason to do it with an astronaut inside, in fact it would be very dangerous. A blowout would give the astronaut the "bends" and quite possibly kill him. If I remember correctly, in space they run about 1/3 to 1/4 of sea level pressure, of pure oxygen, in order to keep the suits limber enough to move around in. Too much pressure inside and the suit would "starfish", splaying out and leaving the astronaut nearly immobile. The first spacewalker was a Russian, who had to release a lot of his air pressure in a hurry so he could get back inside his craft!