reduce

'use strict';

let t = ["abcdefghi"];

let MyA = t[0].split("");

/*
let v = MyA.reduce(function(p,n,i,array){
  //console.log(p,n,i,i%3);
  return (i%3===0) ? (p+"\n"+n) : (p+n);
});
*/

let v = MyA.reduce((p,n,i) => (i%3===0) ? (p+"\n"+n) : (p+n));
console.log(v);