Every national park has a different entrance fee policy, but the three you are going to all have the same fees:
Sequoia and Kings Canyon National Park: $30/vehicle for 7 days
Yosemite National Park:
$30 $25/vehicle for 7 days (it reduces for the winter season to $25 starting November 1)
Grand Canyon National Park: $30/vehicle for 7 days
That adds up to $85 to visit all three parks. In contrast, the America the Beautiful Annual Pass costs $80, and it works for admission at all three parks, plus many more throughout the country (it does not cover additional fees for things like camping or lodging, nor will it work at state parks). So the pass would allow you to save $5. The downside is that the pass is a fixed cost, so you'd wind up spending a little more if you buy the pass in advance and then don't end up visiting all three parks for any reason.
Enjoy your trip!