Why do lasers require at least 3 energy levels?

I don’t yet understand why 2 energy levels aren’t enough as they can still emit photons (even when B12 and B21 are equal). By the way, I’m talking about stimulated emission.

Albert Einstein showed that rate of stimulated emissions is equal to that of stimulated absorptions. That is, a photon of energy $$h \nu$$ has equal probablity to cause either absorption or stimulated emission. Suppose there are only two levels;one ground state and one metastable state. Then proportion of stimulated emissions will be equal to that of absorptions. Therefore no light amplification will occur. Instead if we are exciting atom to a higher energy state above metastable state, the atoms at higher state will eventually fall to metastable state causing population inversion. In this case, number of stimulated emissions will be greater that of absorptions. Then we will have light amplification