IntelliJ and Git: How to see diffs between a commit and two older commit?

You can select any number of commits in the git log window (using shift/ctrl and click or cursor keys) and the right-hand pane will show the cumulated differences.

In IntelliJ, there is no command or action to compare exact revisions, unfortunately. Here are couple related requests: https://youtrack.jetbrains.com/issue/IDEA-125616 and https://youtrack.jetbrains.com/issue/IDEA-100431

However, there is a way to see what has changed between two commits. To do so you need to go to the Version control - Log tab and select the entire range between wanted commits (e.g select the later commit, then scroll down to the older commit and click on it with Shift). In the right pane showing changed files you will see all the changes.