A communication method and apparatus, and a system. A first terminal device obtains first information, where the first information includes a first multicast and broadcast service MBS service identifier and first sidelink SL resource information. A mapping relationship exists between the first MBS service identifier and the first SL resource information. A first MBS is an MBS supported by a first relay terminal device, and the first SL resource information is usable for transmitting data of the first MB S to the first terminal device. The first terminal device receives, based on the first SL resource information, the data that is of the first MBS and that is sent by the first relay terminal device. The first terminal device transmits MBS data in a scenario in which a relay terminal device supports an MBS service.