[рони]

Alexodiy,

document.addEventListener("DOMContentLoaded", function() {
    var num = localStorage.getItem("num") || 0,
        today = sessionStorage.getItem("today");
    if(today === "yes") return;
    sessionStorage.setItem("today", "yes");
    num = +num + 1;
    localStorage.setItem("num", ""+num);
    if(num % 3 === 0 ) alert("3");
});