if it were me doing this question, I would first try and create the vectors (1,0,0) (0,1,0) and (0,0,1) from v1 v2 and v3, then the answer should be obvious.
so (v1+v2+v3)/3=(1,0,0) so then a(v1+v2+v3)/3=(a,0,0)
Doing a same thing for the others yields the result. I hope this was clear.