SOURCE

var arr1 = ["a","b"];
var arr2 = ["A","B"];
var arr3 = ["1","2"];

//arr = ["a_A","a_B","a_1","a_2","b_A","b_B","b_1","b_2","A_1","A_2","B_1","b_2"];

//arr = ["a_A_1","a_A_2","a_B_1","a_B_2","b_A_1","b_A_2","b_B_1","b_B_2"];

cp = arrs => arrs.reduce((a, b) => {
    const arr = [];
    a.forEach(i => {
        b.forEach(j => {
            arr.push(i + "_" + j);
        });
    });
    return arr;
});
console.log(cp([arr1, arr2, arr3]))