ok so what makes big 3 kids so powerful (besides the fact that there parents are the most powerful) is that they get powers. now what i mean buy that is that the kids of the other gods have skills like Aries kids have the skill of close range weaponry kids of Athena are very clever and kids of Apollo have the skill of archery. now what makes these things skills is that not all people have them but they can. now when we look at kids of the big 3 we see things like controlling the water,making lightning come out of spears,and raising the dead all of these things are powers not skills. in the world of Percy Jackson there is nothing stopping a mortal from having a skill that all kids of a non big 3 god has. but they cant have the powers of big 3 kids and that is what makes them great and powerful.