How should I install fence panels on posts that aren't level?
For a trivial difference like 3" (large sections of my boundary have at least 3' difference over 24'), I just wouldn't worry about it.  3" in 24' is 1:96, so nobody will notice that the posts aren't quite vertical. so put the posts in vertically, and then tweak the panels slightly so that they aren't quite square.

If you want to be really precise about it, sinking the fence into the ground is probably the best approach.

For a larger difference, you would need to staircase the fence, probably with shorter sections (to minimize the difference over each section).