You only need a 3D capable AVR if you are using the AVR to switch among the HDMI signals. If each device has its own HDMI connection to the TV and a digital audio connection of some sort to the AVR, the TV does the video switching, then the AVR can be used just for the audio.
It is just easier if the AVR is 3D capable. In that situation, each device has an HDMI connection to the AVR, there is one HDMI going from the AVR to the TV, and the AVR does both the audio and video switching.
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