3 blades is to balance the turbine, every time a blade passes the mast it wobbles a bit (or has extra forces on it anyway) - if you have blades opposite each other then the wobble is passed into the opposite blade, so as that blade comes around past the mast it already has a wobble, so wobbles more.... on and on until something goes wrong (or doesen't go wrong, because you have a much heavier blade to compensate... which has it's own down sides)