Obtain a random variable

You can put them in an array and make the classic Math.floor(Math.random() * myArray.length)

var a = 'HFKDJ3';
var b = 'JFI393';
var c = 'KMMMFN';

var arr = [a,b,c];

console.log(arr[Math.floor(Math.random() * arr.length)]);

Here is an example where:

I create an array of texts with the 3 texts. Pressing the button I call the random function that calculates a random number between 0 and 2 and returns the corresponding text. Show an alert with the text returned.

EDITO

I modify the code to use objects so that the function not only returns the text but also the key of the selected value.

var textos = [
  {key: 'a', value: 'HFKDJ3'}, 
  {key: 'b', value: 'JFI393'}, 
  {key: 'c', value: 'KMMMFN'}
];

function aleatorio(){
  var indice = Math.floor((Math.random() * 3));
  var el = textos[indice];
  return el.key + ': ' + el.value;
}

<input type="button" value="aleatorio" onclick="alert(aleatorio());" />