NEED HELP?
WELCOME TO XAPO SUPPORT
If a site is not recognizing your address as a valid address because it starts with a 3 instead of a 1, you will need to reach out to the site to let them know that this is in fact a valid address. They can check the validation of the address themselves on the blockchain, then hopefully they will validate this new multi-sig address for you and all multi-sig addresses in the future.

Also, please feel free to use your old wallet address if you still have it, if that is easier, as all of the payments sent there will still end up in your Xapo Wallet!