A number of years ago, around 2002 I believe, when I first got into PHP and I had my first webserver, there was a "crontab" file that could be modified and set to execute scripts at pre-determined intervals. I'm not sure if 'crontab' is obsolete now, you might want to do a google search.
Alternatively, you can add a 'timestamp' column to your table, and place the code below in a script that you know a user will execute - preferably in the page where you are inserting the values into the table. By the way, the number 259200 is the number of seconds in 3 days. "time" is the name I happened to choose for the timestamp column.
$timestamp = date(U);
$prevtime = $timestamp - 259200;
mysql_query("INSERT INTO table values ('$var1,'$var2',$timestamp)");
mysql_query("DELETE FROM table WHERE time < '$prevtime'");