function slide1(){
document.getElementById('meu-banner').src="img/banner1.jpg";
setTimeout("slide2()", 3000);
}
function slide2(){
document.getElementById('meu-banner').src="img/banner2.jpg";
setTimeout("slide3()", 3000);
}
function slide3(){
document.getElementById('meu-banner').src="img/banner3.jpg";
setTimeout("slide1()", 3000);
}
<body onLoad="slide1()">