What is the exact filename of the ISO you installed? Sounds like you may have downloaded an older one. If you installed the latest ISO released a few days ago, there would be no updates from the stable branch yet.
Neither gtk3-classic nor lib32-gtk3-classic are in the Manjaro repos and have not been for quite some time. They are only available in the AUR (Arch User Repository)–unless you’re using a third-party repository which is unsupported.
Please copy and paste full error messages, not truncated bits you think are relevant.
Please see: