we plan to upgrade Crucible/Fisheye from 2.7.10 to the latest 3.1.1 on Linux. Do we have to upgrade from 2.7->2.8->2.9->3.0, then to 3.1? Or can we skip all the interim versions?
You should be able to upgrade directly but make sure you test it once. Usually, you can skip minor versions.
Also, make sure you read the upgrade notes, even for the versions you are skipping. Especially if you have any customizations.