This is how you can download the entire repository and set it up locally..
$ pwd
/Users/prabath/p2_repo
$ wget -r -l inf http://dist.wso2.org/p2/carbon/releases/3.0.0/
The above will download the entire P2 repo to the local machine.
Once that is done, go to the running carbon instance, Configure --> Features --> Settings --> Add Repository --> Select Location as Local and set it as, /Users/prabath/p2_repo/dist.wso2.org/p2/carbon/releases/3.0.0 and that's it...