  We consider the singular behaviour of tree-level QCD amplitudes when the
momenta of three partons become simultaneously parallel. We discuss the
universal factorization formula that controls the singularities of the
multiparton matrix elements in this collinear limit and present the explicit
expressions of the corresponding splitting functions. The results fully include
spin (azimuthal) correlations up to ${\cal O}(\as^2)$. In the case of
spin-averaged splitting functions we confirm similar results obtained by
Campbell and Glover.
