Isn't a 3xxxxx wallet address the format for one of the contenders for making a bigger block size? May be wrong but I dismember it being for BU.
Bitcoin addresses that start with a 3 are multisignature and have nothing to do with Bitcoin version, they are valid addresses.
https://en.bitcoin.it/wiki/MultisignatureBut these do not work with p2pool, unless you are running changes to include them. I tried sometime back. I think veqtrus has a patch to implement inclusion of them though.