In this paper, we propose a novel transmission scheme for a three-way massive multiple-input multiple-output (MIMO) relay network where three users exchange their data with the help of a decode-and-forward relay station equipped with a very large antenna array. Our proposed scheme needs only two time-slots for the information exchange. More precisely, the three users first send their symbols to the relay. Then, the relay uses the maximum-ratio combining technique to decode all transmitted symbols and simultaneously transmits these symbols to all three users. Each user applies successive cancelation decoding to decode symbols transmitted from other users. We study the sum spectral efficiency of our proposed transmission protocol. We show that the sum spectral efficiency of our proposed scheme increases noticeably compared to the one of the conventional scheme where three time-slots are required to exchange data among the three users, without increasing the system complexity.