Name: Rolf Mertig Date: 09/30/19-02:15:13 PM Z


q3 = -q2 - q1; t =
 Pair[LorentzIndex[k],
   Momentum[q3]] - (-Pair[LorentzIndex[k], Momentum[q2]] -
    Pair[LorentzIndex[k], Momentum[q1]]);
By
Calc[t]
or
ExpandScalarProduct[t]
you will get 0, as desired.

Rolf