I’ve always been an advocate of running Apple’s macOS Combo Updaters instead of the incremental/delta updates that show in the App Store. When macOS 10.14.3 was released I manually downloaded the 10.14.3 Combo Updater from Apple’s site, but it refused to run saying my Mac did not meet the minimum requirements for installation.
After doing some research, I found a lot of other people have reported this same issue on the Apple Discussion Forums. The workaround posted was to install the incremental/delta 10.14.3 update from the App Store, then reapply the 10.14.3 Combo Updater (for good measure). This process worked for me.
I also verified that although it’s possible to get Mojave running on a HFS+ volume (which I did on one of the Macs in my lab), it can’t be updated to 10.14.3, giving a similar “Mac does not meet the minimum requirements for installation” message. The 10.14.3 update apparently can only be run on a Mac running 10.14.x installed on an APFS volume.