JSFiddle - React, Tailwind, and code Playground

JavaScript

function println() {
	for( var i = 0; i < arguments.length; ++i )
    document.body.appendChild(document.createTextNode(arguments[i]));
  document.body.appendChild(document.createElement("br"));
}

var numbers = [
  [1, 2, 3],
  [4, 5, 6],
  [7, 8, 9]
];

var delimiters = [',', '-', 'x'];

numbers.forEach((x, i) => {
  for (var j = 1, l = x.length; j <= l; j+=2 )
  	x.splice(j, 0, delimiters[i]);
})

println( JSON.stringify(numbers) );