We compute the fermionic (n_f) contributions to the flavour
non-singlet structure functions in unpolarized electromagnetic
deep-inelastic scattering at third order of massless
perturbative QCD. Complete results are presented for the
corresponding nf-parts of the three-loop anomalous dimension and the
three-loop coefficient functions for the structure
functions F_2 and F_L. Our results agree with all partial and
approximate results available in the literature. The present calculation
also facilitates a complete determination of the
threshold-resummation parameters B_2 and D_2^DIS of which only the
sum was known so far, thus completing the information required for the
next-to-next-to-leading logarithmic
resummation. We find that D_2^DIS vanishes in the MSbar scheme.