divs = ['promo_1','promo_2','promo_3'];

function hidepromo() {
for (var i=0; i<divs.length; i++)
document.getElementById(divs[i]).style.display = 'none';
}

function showpromo() {
hidepromo(); //hide them all before we show the next one.
var randomDiv = divs[Math.floor(Math.random()*divs.length)];
var div = document.getElementById(randomDiv).style.display ='';

}