It's possible you'll need to buy new hardware, as there is no more 32-bit hardware support in RHEL7.

The RHEL7 Migration Guide product documentation covers many of the differences of RHEL7, but from RHEL6.

However, there are significant differences from RHEL3 to all future RHEL versions. There is no one document which covers all the differences.

I would make a list of functionality which the RHEL3 system provides, then learn how to implement that functionality in RHEL7. If the RHEL3 system does a few simple things like just static httpd or basic permission-less NFS file sharing, the migration will be simple. If the setup is complex, such as central authentication and many services which depend on each other, then the migration will probably not be simple.

Red Hat do offer paid consulting services if you'd like someone to come out and do it for you.