Rollover buttons. I miss the old way

I am useing thikbox code to open iframes out of flash.

import flash.external.ExternalInterface;
button_mc.onRelease = function() {
ExternalInterface.call(“tb_show”,null,“http://www.google.com/?KeepThis=true&TB_iframe=true&height=365&width=640”,false);
};
button_mc2.onRelease = function() {
ExternalInterface.call(“tb_show”,null,“http://www.google.com/?KeepThis=true&TB_iframe=true&height=365&width=640”,false);
};
button_mc3.onRelease = function() {
ExternalInterface.call(“tb_show”,null,“http://www.google.com/?KeepThis=true&TB_iframe=true&height=365&width=640”,false);
};

Now i need to make some button rollovers for my button.

I know i need to set up my timeline in the button movie clips, but how do i tell flash when i rollover button_mc3 move on the time line.

thanks,