A, B, N, C, D, O, E, F, P,?,?,?

The given series may be divided into 2 groups :

I. A, B, C, D, E, F, ?, ? and II. N, O, P, ?

Clearly, the given series consists of two terms of I followed by one term of II.

The missing terms in I are G and H while the missing term in II is Q.