This is taken from a script I use to compress web pages, I broke out the header/cache stuff:
(the style codes aren't working, at least in preview mode, so I'm leaving the formatting alone)
// send the requisite header information and character set
// check cached credentials and reprocess accordingly
header ("cache-control: must-revalidate");
// set variable for duration of cached content
$offset = 60 * 60 * 72; //sec:min:hrs you said 3 days, right?
// set variable specifying format of expiration header
$expire = "expires: " . gmdate ("D, d M Y H:i:s", time() + $offset) . " GMT";
// send cache expiration header to the client broswer
That would probably do the job. Don't know who originally wrote this, it's found on several sites.