bootstrap: content middle

HTML

<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<nav class="navbar nabbar-inverse">
     <div class="container-fluid">
        <!--<img src="Logo.png" alt="Logo" class="navbar-brand">-->
        <ul class="nav navbar-nav hidden-sm hidden-xs">
            <li class="active"><a href="index.php?pageNr=7">Home</a></li>
            <li><a href="index.php?pageNr=5">Chemisch Lab</a></li>
            <li><a href="index.php?pageNr=3">Fysisch Lab</a></li>
            <li><a href="index.php?pageNr=1">Mechanisch Lab</a></li>
        </ul>
    </div>
</nav>

<div class="container-fluid">
    <div class="row mid">
        <div class="col-md-4 text-center"><a href="index.php?pageNr=5">
    <h1>Chemisch Lab</h1></a></div>
        <div class="col-md-4 text-center"><a href="index.php?pageNr=3">
    <h1>Fysisch Lab</h1></a></div>
        <div class="col-md-4 text-center"><a href="index.php?pageNr=1">
    <h1>Mechanisch Lab</h1></a></div>
    </div>
</div>

CSS

.mid {
    margin-top: 15%;
}