Hola !
Te recomiendo que mires los links
http://wiki.centos.org/AdditionalResources/Repositories
http://wiki.centos.org/AdditionalResources/Repositories/CentOSPlus
En el ultimo indican como configurar el repositorio adicional. Puede que te falte algun repo y por eso no consigas el paquete.
Te adjunto lo que me da el comando yum:
# yum list pgadmin3
Setting up repositories
epel 100% |========================
2.1 kB 00:00
rpmfusion-nonfree-updates 100% |======================== 2.8 kB 00:00
dries 100% |======================== 951 B 00:00
rpmfusion-free-updates-te 100% |======================== 2.8 kB 00:00
rpmfusion-nonfree-updates 100% |======================== 2.8 kB 00:00
extras 100% |======================== 1.1 kB 00:00
rpmfusion-free-updates 100% |======================== 2.8 kB 00:00
dag 100% |======================== 1.1 kB 00:00
update 100% |======================== 951 B 00:00
ftp://ftp.inf.utfsm.cl/pub/Linux/CentOS/4.7/os/i386/repodata/repomd.xml: [Errno 4] IOError: [Errno ftp error] timed out
Trying other mirror.
base 100% |======================== 1.1 kB 00:00
addons 100% |======================== 951 B 00:00
Reading repository metadata in from local files
cee878845a4b4bc0267fd58c0 100% |======================== 26 kB 00:01
rpmfusion-: ################################################## 85/85
Added 85 new packages, deleted 0 old in 0.51 seconds
primary.xml.gz 100% |======================== 4.0 MB 00:43
dag : ################################################## 11258/11258
Added 219 new packages, deleted 3 old in 16.66 seconds
Available Packages
pgadmin3.i386 1.8.4-1.el5 epel
Saludos!
