What are some psychological equations in existence today?

I know of Weber-Fechner law and Rescorla-Wagner’s model, but I was sure that there was a third and was told that there were only like 3 in existence.

So does anyone know this third one (that I probably have just forgot at the moment) or any other in existence.

By equations I mean for example Weber-Fechner’s law of the “just noticeable difference”:

dp = k(dS/S)

where dp = difference in perception, k = constant, dS = change in stimulus intensity, S = stimulus at the present.