Yes, Mattermost 3.0 not included in current Gitlab release.
They wrote on their blog, as to why:
Mattermost 3.0 is a major release requiring manual upgrade steps and therefore not included in 8.8 omnibus-gitlab package. Existing GitLab Mattermost users will have to wait until GitLab 8.9 where upgrade to Mattermost 3.1 should be automated. If you're interested in reading more about how this decision was made, check out the issue on GitLab.com. Also the upgrade design for the next release will be discussed in the same issue so please share your feedback there:
https://gitlab.com/gitlab-org/omnibus-gitlab/issues/1286