No it is not possible.
You can sign a message with multiple private keys and thus have multiple signatures, as with a multisig address. However, that would not map to a multisig address but rather three signatures with each one mapping to the three public keys used in the multisig address.
Ok. But, then how come the signing takes place for an outgoing Tx from a multisig address?
Because it isn't the address that signs the transaction, but rather the public keys of the address that sign the transaction. The reason the address can be identified is because the redeemScript of the multisig address is included in the input. The redeemScript is what makes the address, and it includes some more stuff other than the public keys.