Summation of product of three terms in AP Here is an interesting relationship:
$$\sum_{r=1}^n r(r+m)(r+2m)=\left(\sum_{r=1}^n r\right)\left(\sum_{r=1}^{n+2m} r\right)\tag{1}$$
$$\text{i.e.} \;\; 1\cdot(1+m)(1+2m)+2\cdot (2+m)(2+2m)+3\cdot (3+m)(3+2m)+\cdots +n(n+m)(n+2m)=(1+2+3+\cdots+n)(1+2+3+\cdots+(n+2m))$$
which gives the following for the first few values of $m$:
$$\scriptsize\begin{align}
&1\cdot 1\cdot 1+2\cdot 2\cdot2+3\cdot3\cdot3+\cdots+n\cdot n\cdot n&&=(1+2+3+\cdots+n)(1+2+3+\cdots+n)\\
&1\cdot2\cdot3+2\cdot3\cdot4+3\cdot5\cdot7+\cdots+n(n+1)(n+2)&&=(1+2+3+\cdots+n)(1+2+3+\cdots+(n+2))\\
&1\cdot3\cdot5+2\cdot4\cdot6+3\cdot5\cdot7+\cdots+n(n+2)(n+4)&&=(1+2+3+\cdots+n)(1+2+3+\cdots+(n+4))\\ \end{align}$$
This can be proven by expanding both sides and finding that the result is 
$$\frac {n(n+1)}2\cdot \frac {(n+2m)(n+2m+1)}2\tag{2}$$

Question
  Can  the result can be proven without expansion to the closed form but only by manipulating summands and limits, i.e. prove $(1)$ without first expanding to $(2)$?


Some interesting but commonly-known results follow from this. 
Setting $m=0$ gives the "sum of cubes as square of sum of integers" result:
$$\sum_{r=1}^n r^3=\left(\sum_{r=1}^n r\right)^2$$
Setting $m=1$ gives the "discrete integral"
$$\sum_{r=1}^n r^\overline{3}=\frac{\;n^\overline{4}}4$$
where $r^\overline{a}$ is the symbol for the rising factorial.

I will use the identity
$$\sum_{r=1}^n r^3=\left(\sum_{r=1}^n r\right)^2$$
It can be proved without direct expansion. (https://math.stackexchange.com/a/1215805/268334)
\begin{align}
&\;\sum_{r=1}^n r(r+m)(r+2m)\\
=&\;\sum_{r=1}^n [(r+m)^3-m^2(r+m)]\\
=&\;\sum_{r=1}^{n+m}r^3-\sum_{r=1}^mr^3-m^2\sum_{r=1}^n (r+m)\\
=&\;\left(\sum_{r=1}^{n+m}r\right)^2-\left(\sum_{r=1}^mr\right)^2-m^2\sum_{r=1}^n (r+m)\\
=&\;\left(\sum_{r=1}^{n+m}r-\sum_{r=1}^mr\right)\left(\sum_{r=1}^{n+m}r+\sum_{r=1}^mr\right)-m^2\sum_{r=1}^n (r+m)\\
=&\;\left(\sum_{r=1}^n (r+m)\right)\left(\sum_{r=1}^{n+m}r+\sum_{r=1}^mr\right)-m^2\sum_{r=1}^n (r+m)\\
=&\;\left(\sum_{r=1}^n (r+m)\right)\left(\sum_{r=1}^{n+m}r+\sum_{r=1}^mr-m^2\right)\\
=&\;\left(\sum_{r=1}^n r+mn\right)\left(\sum_{r=1}^{n+2m}r-\sum_{r=1}^m(r+m+n)+\sum_{r=1}^mr-m^2\right)\\
=&\;\left(\sum_{r=1}^n r+mn\right)\left(\sum_{r=1}^{n+2m}r-m(m+n)-m^2\right)\\
=&\;\left(\sum_{r=1}^n r+mn\right)\left(\sum_{r=1}^{n+2m}r-\sum_{r=1}^m(r+m+n)+\sum_{r=1}^mr-m^2\right)\\
=&\;\left(\sum_{r=1}^n r+mn\right)\left(\sum_{r=1}^{n+2m}r-m(2m+n)\right)\\
=&\;\left(\sum_{r=1}^n r\right)\left(\sum_{r=1}^{n+2m}r\right)+mn\sum_{r=1}^{n+2m}r-m(2m+n)\sum_{r=1}^nr-m^2n(2m+n)\\
=&\;\left(\sum_{r=1}^n r\right)\left(\sum_{r=1}^{n+2m}r\right)+\frac{1}{2}mn(n+2m)(n+2m+1)\\
&\qquad-\frac{1}{2}m(2m+n)n(n+1)-m^2n(2m+n)\\
=&\;\left(\sum_{r=1}^n r\right)\left(\sum_{r=1}^{n+2m}r\right)+\frac{1}{2}mn(n+2m)(n+2m+1-n-1-2m)\\
=&\;\left(\sum_{r=1}^n r\right)\left(\sum_{r=1}^{n+2m}r\right)
\end{align}
