What is third harmonics in ac circuits?

It depends on the frequency in the expression of the function. A third harmonic would mean that the frequency is 3 omega, thus the circuit will consist only of the quantities that have 3 omega in the expression. For example if you have :

e(t)=5sin(3ωt+π/4) and is(t)=7sin(ωt+π/4), in the first harmonic the e(t) quantity

will disappear from the circuit while in the third harmonic the is(t) quantity will be neglected.