Random images on load in this slider

I've got this slider from codepen, and I wanted to showcase some work on it later, for example the first slide will be 'Photography' next one can be 'Illustrations' and so on.

I want to have 3 images that are random chosen on each refresh (I have now one that shows but there are 2 other written in comment in HTML).

Here is my code.

let dots = 4;
let sliderElem = document.querySelector(".slider");
let dotElems = sliderElem.querySelectorAll(".slider__dot");
let indicatorElem = sliderElem.querySelector(".slider__indicator");

Array.prototype.forEach.call(dotElems, dotElem => {
        dotElem.addEventListener("click", e => {
                let currentPos = parseInt(sliderElem.getAttribute("data-pos"));
                let newPos = parseInt(dotElem.getAttribute("data-pos"));

                let newDirection = newPos > currentPos ? "right" : "left";
                let currentDirection = newPos < currentPos ? "right" : "left";

                indicatorElem.classList.remove(`slider__indicator--${currentDirection}`);
                indicatorElem.classList.add(`slider__indicator--${newDirection}`);
                sliderElem.setAttribute("data-pos", newPos);
        });
});

.slider {
  position: relative;
  width: 100%;
  height: 100%;
  overflow: hidden;
}
.slider__slides {
  position: relative;
  width: 400%;
  height: 100%;
  -webkit-transition: -webkit-transform 0.3s cubic-bezier(0.51, 0.92, 0.24, 1);
  transition: -webkit-transform 0.3s cubic-bezier(0.51, 0.92, 0.24, 1);
  transition: transform 0.3s cubic-bezier(0.51, 0.92, 0.24, 1);
  transition: transform 0.3s cubic-bezier(0.51, 0.92, 0.24, 1), -webkit-transform 0.3s cubic-bezier(0.51, 0.92, 0.24, 1);
  will-change: transform;
}
.slider[data-pos="0"] .slider__slides {
  -webkit-transform: translateX(0%);
          transform: translateX(0%);
}
.slider[data-pos="1"] .slider__slides {
  -webkit-transform: translateX(-25%);
          transform: translateX(-25%);
}
.slider[data-pos="2"] .slider__slides {
  -webkit-transform: translateX(-50%);
          transform: translateX(-50%);
}
.slider[data-pos="3"] .slider__slides {
  -webkit-transform: translateX(-75%);
          transform: translateX(-75%);
}
.slider__slide {
  float: left;
  width: 25%;
  height: 100%;
}
.slider__slide img {
  width:50%;
  display: block;
  margin: 0 auto;
}
.slider__dots {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  position: absolute;
  bottom: 1.5em;
  left: 50%;
  -webkit-transform: translateX(-50%);
          transform: translateX(-50%);
}
.slider__dot, .slider__indicator {
  display: block;
  margin: 0 0.5em;
  width: 1em;
  height: 1em;
  background: rgba(255, 255, 255, 0.5);
  border-radius: 100px;
}
.slider__indicator {
  position: absolute;
  background: white;
  width: auto;
}
.slider__indicator--left {
  -webkit-transition: left 0.3s cubic-bezier(0.51, 0.92, 0.24, 1.15), right 0.3s 0.1s cubic-bezier(0.51, 0.92, 0.24, 1.15);
  transition: left 0.3s cubic-bezier(0.51, 0.92, 0.24, 1.15), right 0.3s 0.1s cubic-bezier(0.51, 0.92, 0.24, 1.15);
}
.slider__indicator--right {
  -webkit-transition: left 0.3s 0.1s cubic-bezier(0.51, 0.92, 0.24, 1.15), right 0.3s cubic-bezier(0.51, 0.92, 0.24, 1.15);
  transition: left 0.3s 0.1s cubic-bezier(0.51, 0.92, 0.24, 1.15), right 0.3s cubic-bezier(0.51, 0.92, 0.24, 1.15);
}
.slider[data-pos="0"] .slider__indicator {
  left: 0em;
  right: 6em;
}
.slider[data-pos="1"] .slider__indicator {
  left: 2em;
  right: 4em;
}
.slider[data-pos="2"] .slider__indicator {
  left: 4em;
  right: 2em;
}
.slider[data-pos="3"] .slider__indicator {
  left: 6em;
  right: 0em;
}

.center {
  display: -webkit-box;
  display: -ms-flexbox;
  display: flex;
  height: 100vh;
  -webkit-box-pack: center;
      -ms-flex-pack: center;
          justify-content: center;
  -webkit-box-align: center;
      -ms-flex-align: center;
          align-items: center;
  background: #333;
}

.slider {
  max-width: 600px;
  max-height: 400px;
  box-shadow: 0 0 20px rgba(0, 0, 0, 0.8);
}
.slider__slide:nth-child(1) {
  background: #309954;
}
.slider__slide:nth-child(2) {
  background: #FFBD3C;
}
.slider__slide:nth-child(3) {
  background: #F8593E;
}
.slider__slide:nth-child(4) {
  background: #4086FA;
}

<div class="center">
        
        <div class="slider" data-pos="0">
                <div class="slider__slides">
                        <div class="slider__slide">
                          <img src="http://ift.tt/2udibpY">
        
        <!-- Second image that has a chance of showing upload load
        <img src="http://ift.tt/1V99Hqw">
      -->
      
      <!-- Third
        <img src="http://ift.tt/1V99Hqw">
      -->
        
        
        
        
                        </div>
                        <div class="slider__slide"></div>
                        <div class="slider__slide"></div>
                        <div class="slider__slide"></div>
                </div>
                <div class="slider__dots">
                        <a href="#" class="slider__indicator"></a>
                        <a href="#" class="slider__dot" data-pos="0"></a>
                        <a href="#" class="slider__dot" data-pos="1"></a>
                        <a href="#" class="slider__dot" data-pos="2"></a>
                        <a href="#" class="slider__dot" data-pos="3"></a>
                </div>
        </div>
        
</div>

I beleive in order to have the random images I need to have a var saved in javascript this way, is that correct?

 var imgs = [
        'http://ift.tt/2udibpY',
        'http://ift.tt/1V99Hqw',
        'http://ift.tt/1V99Hqw'
    ];

To get a random one:

var img_index = Math.floor( Math.random()*imgs.length );

But how do I know integrate this in HTML?