Adrien, if you want three sets of stereo speakers in three different locations playing the same material, the simplest and least expensive option probably is to just get one of the 508 Onkyo refurbs from Accessories4less for about $180. Select the all-channel stereo mode, connect the two inside sets to main and side surround terminals and the outside set to the back surround terminals.

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Enjoy the music, not the equipment.