divs side by side

HTML

<div id="outer">
    <div class="inner1">1</div>
    <div class="inner2">2</div>
    <div class="inner3">3</div>
</div>

CSS

#outer {
    width: 90%;
    height: 90%;
    margin: 5%;
    overflow: auto;
    background-color: red;
}

.inner1 {
    float: left;
    width: 150px;
    height: 150px;
    margin-right: 20px;
    background-color: blue;
}
.inner2 {
    float: left;
    width: 150px;
    height: 150px;
    margin-right: 20px;
    background-color: blue;
}
.inner3 {
    float: left;
    width: 150px;
    height: 150px;
    
    background-color: blue;
}