Splitting nested arrays [duplicate]

I would like to split nested arrays into groups of 3 (in javascript):

e.g.

let array = [[123],[456],[789],[321],[654],[987]];

I would like split this into a new nested array, i.e.

new_array = [
              [
                [123],[456],[789]
              ]
              [
                [321],[654],[987]
              ]
            ]

let array = [[123],[456],[789],[321],[654],[987]];

let new_arr = [];
for (i=0; i<array.length; i+2){
    new_arr.push(array[i]);
    i++;
    new_arr.push(array[i]);
    i++;
}

When I log this out to the console, there is no grouping, it just added all the nested array to a new array.