Do you especially need the Mate desktop? You’ll find armhf and arm64 versions of Xubuntu 18.04 for the Raspberry Pi on this thread AArch64 on Raspberry Pi 2 (rev 1.2), 3B, 3B+ . The Pi 3B+ works out of the box.
If you really want Mate, then there are multiple threads on here that explain how to get it on the 3B+. No Mate image for 18.04 exists (yet), but you can upgrade from 16.04.