Thank you, Peter Robinson, your instructions worked beautifully.
Perhaps this will not work with the aarch64 version used by Tomáš Frolík,
but you already explained in an earlier post why there is little reason
to use that on a Raspberry Pi.
I used the following to install on a SD card:
--media=/dev/sde --norootpass --resizefs --selinux=off
and copied the dtb file to bcm2837-rpi-3-b-plus.dtb as described, and
then successfully booted my Raspberry Pi 3B+.
After completion of the first-boot dialog, things pretty much fell
apart. I saw the expected pointer on my screen, but the machine stuck
before it displayed the graphical login screen. There was no response to
my attempt to switch to another terminal with Ctrl-Alt-F4 or Alt-F4.
I mounted my SD card on my regular Fedora workstation, then manually
changed the default target to multi-user.target. This worked much
better, I could log in and discover the RPi wired Ethernet is not
Bypass this problem with a USB Ethernet dongle (which is automatically
configured without any problems), and I can run "dnf upgrade" which
mostly succeeds (there is a problem with libvirt-daemon, but that occurs
with the RPi 3B also.)
Reboot, and I now have an (except for libvirt-daemon) up-to-date F28 on my
Raspberry Pi 3B+.
No graphical login and no wired Ethernet are problems to be solved, but
at least there is a path to get a current F28 environment on the 3B+ for
those who want to work on this machine.