It was necessary in fulfilment of the divine purpose. See Mat 26:54; Heb 8:3; Luk 24:26.
Suffer
This first announcement mentions his passion and death generally; the second (Mat 17:22, Mat 17:23), adds his betrayal into the hands of sinners; the third (Mat 20:17-19), at length expresses his stripes, cross, etc.
Elders and chief priests and scribes
A circumstantial way of designating the Sanhedrim, or supreme council of the Jewish nation.