In order to build and install pgadmin3 on RHEL 5, you will need wxGTK RPMs. In order to install wxGTK to RHEL 5, first please visit
http://download.fedora.redhat.com/pub/epel/5/i386/repoview/epel-release.html

and install epel-release rpm. Then install the wxGTK rpm from epel repository. If you want to rebuild SRPM, you will also need wxGTK-devel RPM.