list posts of one category in header.php file when is_single view

<?php if (is_single()){ echo wp_get_archives('type=postbypost'); } ?>

I only have one category of posts that uses single.php file. So, when a post is view, I want a list generated of all other posts in that category. I get the list with all{every category} posts with the function above, but I only need cat 3.

Any ideas would be great.
Thanks for helping in advanvce