Installing packages that belong to an older version of your distribution can be problematic and may make your system unusable. If you really want to use this approach:
Check if the missing packages are available from the same sourcehttp://mirror.centos.org/centos/7/os/x86_64/Packages/and install these. The dependencies might depend on more old packages, so you might need to reiterate untill you fulfill all recursive dependencies.
The resulting program will probably depend on several old library versions, so it will probably not run on other Centos 8 systems without manually installing the library dependencies as you did on the build system. (except for the developer libraries)
In my opinion, building Qt3 from source based on the libraries of your Centos 8 system, if possible, is a better option. This will avoid the problems that can arise from using packages that are not intended for your distribution.
Statically linking your local Qt3 libraries, if possible, would allow running the program on other Centos 8 systems. Otherwise you would have to package your program with the Qt3 shared libraries.