将鼠标悬停在另一个 id 的 id 和属性更改上

*{
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}

html,
body {
    margin: 0;
}

header {
    width: 100%;
    height: 60px;
    background-color: #323232;
    position: fixed;
    text-align: center;
}

.header-links {
    height: 60px;
    position: absolute;
    display: inline-block;
    margin-left: -194px;
}

.header-links>ul {
    margin-top: 0px;
    float: left;
}

.header-links>ul>li {
    display: inline-block;
    width: 100px;
    height: 60px;
	transition: .3s;
}

.header-links>ul>li>a {
    display: block;
    margin-top: 40px;
    font-family: 'Poiret One', cursive;
    text-decoration: none;
    color: #fff;
}


.header-links>ul>li.triangle{
	position: absolute; top: 100%; left: 100px;
	display: block;
	width: 0;
    height: 0;
    border-top: 30px solid #323232;
    border-right: 50px solid transparent;
    border-left: 50px solid transparent;    
    margin: 0 auto;   	
	transition: .3s;
}

.header-links>ul>li:hover:first-child ~ .triangle{
	left: 0;
}
.header-links>ul>li:hover:nth-child(2) ~ .triangle{
	left: 100px;
}
.header-links>ul>li:hover:nth-child(3) ~ .triangle{
	left: 200px;
}

<header>
    <div class="header-links">
        <ul>
            <li id="link-main"><a href="">Ссылка</a></li>
            <li><a href="" class="active">Ссылка</a></li>
            <li><a href="">Ссылка</a></li>
			<li class="triangle"></li>
        </ul>
    </div>
</header>

html,
body {
  margin: 0;
}
ul,
li {
  list-style: none;
}
header {
  width: 100%;
  height: 60px;
  background-color: #323232;
  position: fixed;
  text-align: center;
}
.header-links {
  height: 60px;
  display: inline-block;
}
.header-links > ul {
  margin-top: 0px;
  display: flex;
  justify-content: center;
  position: relative;
}
.header-links > ul > li {
  display: inline-block;
  width: 100px;
  height: 60px;
  text-align: center;
}
.header-links > ul > li > a {
  display: block;
  margin-top: 40px;
  font-family: 'Poiret One', cursive;
  text-decoration: none;
  color: #fff;
}
#header-triangle {
  width: 0;
  height: 0;
  border-top: 30px solid #323232;
  border-right: 50px solid transparent;
  border-left: 50px solid transparent;
  position: absolute;
  left: 50%;
  top: 60px;
  transform: translatex(-30px);
  transition: .2s;
}
.header-links > ul > li:hover:nth-child(1) ~ #header-triangle {
  left: calc(50% - 100px);
}
.header-links > ul > li:hover:nth-child(3) ~ #header-triangle {
  left: calc(50% + 100px);
}

<header>
  <div class="header-links">
    <ul>
      <li id="link-main"><a href="">Ссылка</a>
      </li>
      <li><a href="">Ссылка</a>
      </li>
      <li><a href="">Ссылка</a>
      </li>
      <li id="header-triangle">
      </li>
    </ul>
  </div>
</header>