Pass variables between different .js files; one is inside an iframe

You can not "pass" variables through file references. You would need to add code to pass data from the parent frame to the iframe.

If the variable is global it is

//from the iframe
var theVariable = window.parent.yourVaraibleName;

//from the parent
var theVariable = document.getElementById("iframeId").contentWindow.yourVaraibleName;

Why not using jQuery cookies to pass the variables? Even within the multiple pages. Once you pass the variable you can destroy the cookie.

You cannot pass variables from one js file to the other Javascript variables are stateless so they wont be retained. If you are using .Net then you can make use of Session variables to solve this purpose. If you use MVC you can go for viewbag or viewdata. If its a must then declare some variable in the homepage, then assign the value to be passed to the variable in home page and then call the function in 3.js passing this parameter.