We present an efficient two-step entanglement concentration protocol (ECP) for three-level atoms trapped in one-sided optical micro-cavities in an arbitrary three-particle less-entangled W state, using the coherent state input-output process in low-Q cavity quantum electrodynamics system. In each step of the new proposed protocol, one of the three remote users prepares the auxiliary coherent optical pulses to perform cavity input-output process and then utilizes the standard homodyne measurement to discriminate the final outgoing coherent states. When both of the two steps are successful, remote parties can deterministically concentrate the less-entangled W state atoms to a standard maximally entangled W state. Compared with previous ECPs for W state, this protocol has some advantages and can be widely used in current quantum repeater and some quantum information processing tasks.