[boomboomsubban]

This seems to be the original source https://physicsworld.com/a/the-greatest-equations-ever/ but it doesn't actually rank the equations. The other source commenting on that does, but only the sample is available on Google Scholar. From that, first is Euler's identity, second is Maxwell's four electromagnetic field equations, and third isn't in the sample. The NYT article also commenting on it https://archive.ph/H7ujx suggests the theory of relativity, F=ma, or amusingly 1+1=2.

[BlueTemplar]

Maxwell's even better as a single equation :

http://www.av8n.com/physics/maxwell-ga.htm