* {
  box-sizing: border-box;
  padding: 0;
  margin: 0;
  text-decoration: none;
}
html {
  scroll-behavior: smooth;
}
body {
  display: flex;
  flex-direction: column;
  height: 100%;
}
.box1 {
  background-color: antiquewhite;
  height: 400px;
  width: 300px;
}
.box2 {
  background-color: yellow;
  height: 400px;
  width: 300px;
  margin-top: 50px;
}
.box3 {
  background-color: red;
  height: 400px;
  width: 300px;
  margin-top: 50px;
}
.slider {
  display: flex;
  flex-direction: column;
  position: fixed;
  left: 320px;
  top: 50%;
  text-align: center;
}
a {
  color: #fff;
  height: 40px;
  width: 50px;
  padding: 7px;
}
a:nth-child(1) {
  background-color: pink;
}
a:nth-child(2) {
  background-color: blue;
}
a:nth-child(3) {
  background-color: pink;
}