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The deployment of relays has been recognized as a valid way to increase spectral efficiency and coverage in wireless systems. In this work we consider a network where no communication is allowed from the source to the destination and transmission is performed only with the assistance of three relays operating in half-duplex mode. Moreover, time is divided in two phases and only one relay is transmitting during odd phases, while the other two are transmitting during even phases. The two relays transmit signals belonging to different scaled and rotated QAM constellations. However, in order to simplify the receiver, the two signals combine through the channels to a symbol belonging to a larger QAM constellation. Then, we develop a practical algorithm to perform power and time-sharing optimization in order to maximize spectral efficiency. Numerical results show that our proposal outperforms the scenario where cooperative relays employ the classical adaptive decode-and-forward and indeed approaches the spectral efficiency obtained when relays employ Slepian-Wolf encoding.