Embodiments provide a data transmission method, a device, and a system. The method includes: acquiring, by user equipment, a first preamble corresponding to a cell set of a primary timing advance group of a first base station and a second preamble corresponding to a cell set of a secondary timing advance group of a second base station; sending the first preamble to the first base station, to initiate random access to the first base station according to the first preamble; and sending the second preamble to the second base station, to initiate random access to the second base station according to the second preamble.