I’ve figured out how to get near-instant stereoscopic 3D movies in Motion 3. Download this Motion file which has several settings about right. Activate the horizontal two-up view (the second option in the View Layouts menu under the Timing button). You may need to assign the cameras to the appropriate panes for a cross-eyed view (left on the right, right on the left). Command-option-/ hides the 3D view info, so hide it in each pane.
Now go cross-eyed to unify the two panes into a true stereoscopic 3D view. You can’t press play, because only the active pane will update. You can step frame-by-frame. You can drag the playhead. Enjoy!
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