250, 3, 7, 1, 1 aren't 3 parts, more like 5 parts.
371, 1, 250 are 3 parts.
The right combination for that case is 3 * 7112 * 50 = 1066800

not the most beautiful but only one reduce instead of two.

yes, there is splitted into 3 parts because the multiplication of the other 2 parts is the same, for example:
[250][3,7,1,1] => 250 * 3 * 7 * 1 * 1 = 5,250;
[3,7][250][1,1] => 3 * 7 * 250 * 1 * 1 = 5,250;
See what I mean? Commutative Property of Multiplication can be applied.

yes, but is O(n^2), includes also search in the whole used array. forEach is also slower than for

You should split the number into three parts

for 3711250, I'm generating the following partitions, I'm I missing something?

[71][3,1,2,5,0]
[11][3,7,2,5,0]
[12][3,7,1,5,0]
[25][3,7,1,1,0]
[50][3,7,1,1,2]
[371][1,2,5,0]
[711][3,2,5,0]
[112][3,7,5,0]
[125][3,7,1,0]
[250][3,7,1,1]
[3711][2,5,0]
[7112][3,5,0]
[1125][3,7,0]
[1250][3,7,1]
[37112][5,0]
[71125][3,0]
[11250][3,7]```
Expected: 1066800, instead got: 236250

I think it would be great if you could add an example with a bigger entry