Yeah, it might not be the easiest to accomplish an integrated form solution that will filter and sort in this instance…
You could otherwise use a Javascript driven form for a Category parameter to set the URL (to add the category segment into the URL) so it looks like this:
http://www.yoursite.com/user/users/C5
Then dynamically sort with the other two dropdowns as a seperate form
You’ll need to set your orderby and sort form return URL to have a {segment_3} built into it to retain the Category number in the URL.
I’ve tried testing this out a bit on my test site, and the Users loop seems to react to the dynamic category URL.