Three different examples of three consecutive triangular numbers whose sum is a perfect square for n > or equal to 20

Three different examples of three consecutive triangular numbers whose sum is a perfect square for n > or equal to 20. (In other words their sum must be greater than or equal to 400 and must be a perfect square).

I can use a program such as maple to write a script for it but I don't know where to even begin.

Using Joffan's suggestion I wrote out

$\frac{1}{2} j(j+1) + \frac{1}{2} k(k+1) + \frac{1}{2} l(l+1) = m^2 $

I am just not certain how I can translate this into maple's input language.