In CentOS 6 / RHEL6, nfs client is defaulted to mount as version 4. For any reason one would want to mount nfs as version 3, here is the options to do so
[root@dumphost ~]# showmount -e nfshost
Export list for nfshost:
[root@dumphost ~]# mount -o vers=3 nfshost:/nfs /mnt
[root@dumphost ~]# mount
nfshost/nfs on /mnt type nfs (rw,vers=3,addr=10.1.0.2)
In any case you want version 3 be set as default, uncomment the option Defaultvers from /etc/nfsmount.conf and change it from 4 to 3.
[root@dumphost ~]# grep Defaultvers /etc/nfsmount.conf