Multiple Loops with in_category tag

I’ve got a template set up the way I want, and it seems to be working. But I’ve had to use conditional tags in a way opposite to what the WP documentation specifies, and I’d appreciate anyone shedding light on the matter.

What I wanted was to have multiple category-specific post loops on one page (index.php, say). I put this in each loop:

<?php if (in_category('1')) continue;?>

And after each endwhile, I have this (is it necessary? It doesn’t seem to make a difference.):

rewind_posts();

This works, sort of. What it ended up doing is excluding everything in category one instead, or simply including posts in all categories.

To get a post loop to show only posts in category 1, I had to use:

<?php if (in_category('2') || in_category('3')) continue;?>

or

<?php if (!in_category('1')) continue;?>

to exclude all other categories.

Isn’t in_category() supposed to specify which categories to include rather than exclude?

Thanks.