I've tried multiple apps, and the crop keeps coming out one pixel off (e.g. 3000x3001). I need perfectly square pictures for album cover art. I can't find any solution in settings.
There probably are none, because of the way the picture is built. Within 0.3%, it's square. Copy the artwork to a PC and use a program like Paint.net to delete that last row of pixels. (It's a lot of work, but perfection is never easy.)
Cover art at 3000x3000 pixels? Really? Why so large?
I typically make mine 600X600 at 72px/inch, which is plenty big. Then I save at 50% compression. This is more than good enough for phones, tablets, or vehicle media center displays.