Why is the nuclear weapon/entity symbol the three pronged one that we see?

Answer 1

The three-bladed radiation warning symbol, as we currently know it, was "doodled" out at the University of California Radiation Laboratory in Berkeley sometime in 1946 by a small group of people

Yellow was picked because it indicated hazard and black was chosen because black and yellow is the most common color combination in the US.

The trefoil was picked because it is a simple, easy to understand design.