LHS=mC0+mC1(x)+mC2(x^2)+...+m+3Cr (x^r)
RHS=[(1+x)^m=mC0+mC1(x)+mC2(x^2)+...+mCr-3 (x^r-3)+mCr-2(x^r-2)+mCr(x^r-1)+mCr(x^r)]((1+3x^2+3x+x^3)
By comparing coefficients of x^r,
m+3Cr=mCr+3(mCr-1)+3(mCr-2)+mCr-3 #
Edit: Wish i saw your post Tim, i would have never done any of that lol