How to show different image on JavaScript onclick and display the same image on 3rd or 4th click

window.onload = choosePic;
var myPix = new Array("img1.jpg", "img2.jpg", "img3.jpg");

function choosePic() {
  var randomNum = Math.floor(Math.random() * myPix.length);
  document.getElementById("myPicture").src = myPix[randomNum];
  document.getElementById("myPicture2").src = myPix[randomNum];
  document.getElementById("myPicture3").src = myPix[randomNum];
}

<img src="img.jpg" width="100" height="100" id="myPicture" alt="some image" />
<img src="img.jpg" width="100" height="100" id="myPicture2" alt="some image" />
<img src="img.jpg" width="100" height="100" id="myPicture3" alt="some image" />
<button id="btn" onclick="choosePic()">Click Hear</button>

I want to show 3 different images from the array. and on 4th or 5th click want to show the same pic.