Combonatorial Proof for C(n+2, 3) = (1* n) + (2 * (n-1)) …. + (n*1)

Just as the title say, I need to find a combinatorial proof for the question

C(n+2,3) = 1⋅n+2⋅(n−1)+3⋅(n−2)+⋯+(n−1)⋅2+n⋅1

Any help is appreciated,
Thanks!