simples Slide Show

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8"/>
    <title>Slide Show</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <link rel="stylesheet" type="text/css" href="style.css">
<style>
#main{
    max-width: 1080px;
    margin: 0 auto;
}
img{
    max-width: 100%;
}
</style>
</head>
<body onload="slide1()">
    <div id="main">
        <img src="banner1.jpg" alt="slide show" id="banner">
    </div>
<script src="arquivo.js"></script>
</body>
</html>
var intervalo = 3000;

function slide1(){
    document.getElementById("banner").src="banner1.jpg";
    setTimeout("slide2()",intervalo);
}
function slide2(){
    document.getElementById("banner").src="banner2.jpg";
    setTimeout("slide3()",intervalo);
}
function slide3(){
    document.getElementById("banner").src="banner3.jpg";
    setTimeout("slide1()",intervalo);
}