  We study a three-point correlator of the three heavy vertex operators
representing the circular winding string states which are point-like in AdS_5
and rotating with two spins and two winding numbers in S^5. We restrict
ourselves to the case that two of the three vertex operators are located at the
same point. We evaluate semiclassically the specific three-point correlator on
a stationary splitting string trajectory which is mapped to the complex plane
with three punctures. It becomes an extremal and 4d conformal invariant
three-point correlator on the boundary. The marginality condition of the vertex
operator is discussed.
