Embodiments of the present invention disclose a data transmission method and system, and a device. The method includes: obtaining, by a transmission device, configuration information sent by a base station; receiving, by the transmission device, scheduling assignment SA signaling sent by at least one sending device; receiving, by the transmission device, first data, where the first data is the data indicated by the SA signaling that is received by the transmission device from the at least one sending device; and sending, by the transmission device, relay data, where the relay data is one or more pieces of data of the first data. The embodiments of the present invention are used for D2D communication.