This package conflicts with provided ./rhel-8-for-x86_64-appstream-rpms/Packages/c/cmake-3.11.4-3.el8.x86_64.rpm which provides cmake3 and contains the packages
/usr/bin/ccmake3 /usr/bin/cmake3 /usr/bin/cpack3 /usr/bin/ctest3 /usr/lib/.build-id
The only way I can see this being allowed in EPEL would be for the EPEL cmake3 package being called cmake3.17 and not conflicting with the above files.
What I did to test in the end was to create a small virtual machine and install the packages from the existing setup. I then copied over /etc/nagios. I updated to the newest nagios and ran a nagios -v /etc/nagios/nagios.cfg to see what broke. For our setup we had customized things in a way which 'worked' in previous versions but didn't in the new version. I fixed those up and then edited my nagios.cfg differences into the nagios.cfg.rpmnew so I got the additional configs that the new version wanted.