bootstrap.css should contain all the CSS you need to use Bootstrap in your project.
The files bootstrap-reboot.css and bootstrap-grid.css are cut down versions containing just the necessary styles for reboot and Bootstrap’s flexbox grid respectively. They are to be used if you don’t want to include the entirety of Bootstrap in your project and will only be making use of either of these features.
The docs explain it here: https://getbootstrap.com/docs/4.1/getting-started/contents/#css-files