Update 3 is a roll up of bug fixes on top of Team Foundation Server 2018 Update 2. You can see a list of the bugs that were fixed in the release notes. This RC is a go-live release, so you can install it on your production servers.
This is our only planned release candidate for TFS 2018.3. Please report any problems on Developer Community or call customer support if you need immediate help. We’d love for you to install TFS 2018.3 RC and we’re looking forward to your feedback.