We have multiple repositories in Nexus (i.e., releases, snapshot and site). All 3 repos are under public group and users uses the same credentials to access all these repositori
It looks like you mistaken things here. The given credentials and id's are for the distributionManagement and not for the access to the Nexus in this case. Apart from that you need three different username, password combinations cause you have three possible things releases, snapshots and site. So not a big deal.
Aprt from that i would suggest to upgrade Maven into Maven 3.X line cause Maven 2.2.1 is a little bit out of date.