Hallo,
hier meine überarbeitete Version, die in allen Browsern funktioniert. Zwar nicht so schön wie dein Beispiel aber immerhin.
section{
width: 500px;
margin: 0 auto;
}
.form_suche {
width: 100%;
margin-top: 3em;
padding: 1em;
box-sizing: border-box;
text-align: center;
}
.div_oben {
background-color: #efefef;
padding: 1em;
width: 100%;
}
.select_1 {
display: inline-block;
width: 30%;
}
.select_2 {
display: inline-block;
width: 30%;
}
.select_3 {
display: inline-block;
width: 30%;
}
.btn_suche {
width: 50%;
background-color: #efefef;
padding-bottom: 10px;
margin: 0 auto;
}
<section>
<form name="form1" method="post" action="" class="form_suche">
<div class="div_oben">
<div class="select_1">
<select></select>
</div>
<div class="select_2">
<select></select>
</div>
<div class="select_3">
<select></select>
</div>
</div>
<div class="btn_suche">
<button type="submit">Suchen</button>
</div>
</form>