There's a Craft 3 port of Category Sources in the
v2 branch for the repo. It doesn't appear to be published to Packagist and the Plugin Store, which means you'll need to add the URL for the plugin's Git repository to your project's
composer.json file in order for Composer to find it.
Add the following to the root of your
You might also need to add the following to
Then, you can do
composer require craftcms/category-sources to actually download the plugin.
Note: I haven't tested this plugin myself, and the fact that it isn't released probably means it's not completely ready for primetime (it might not even work at all, with recent versions of Craft 3). Proceed with caution; you might also want to get in touch with Pixel & Tonic to hear if the plugin is likely to get a proper release anytime soon.