http://monerotoruzizulg5ttgat2emf4d6fbmiea25detrmmy7erypseyteyd.onion/resources/user-guides/multisig-messaging-system.html
(Often there will be indeed 3 distinct people controlling the 3 wallets, but not always of course.) The MMS also manages a single list of messages per wallet: All messages you send, plus all messages you receive. While the list of authorized signers is the same in all involved wallets, those messages of course differ.