  In conformal field theory in Minkowski momentum space, the 3-point
correlation functions of local operators are completely fixed by symmetry.
Using Ward identities together with the existence of a Lorentzian operator
product expansion (OPE), we show that the Wightman function of three scalar
operators is a double hypergeometric series of the Appell $F_4$ type. We extend
this simple closed-form expression to the case of two scalar operators and one
traceless symmetric tensor with arbitrary spin. Time-ordered and
partially-time-ordered products are constructed in a similar fashion and their
relation with the Wightman function is discussed.
