What kind of integer? A Unix timestamp? If so, you'll need to use this WHERE statement:
WHERE releasedate <= (UNIX_TIMESTAMP() - 60*60*24*3)
EDIT: Now that I think about it... and maybe I'm understanding you wrong... but I believe you've got your logic reversed. You want 'releasedate' values that are greater than or equal to (e.g. the same as or more recent) the Unix timestamp of 3 days ago. As such, your query should probably look like this:
WHERE releasedate >= (UNIX_TIMESTAMP() - 60*60*24*3)