code: protected void capture3DScreenShot(GVRRenderTarget renderTarget, boolean isMultiview) {
          if (mScreenshot3DCallback == null) {
              return;
          }
          final Bitmap[] bitmaps = new Bitmap[6];
          renderSixCamerasAndReadback(mMainScene.getMainCameraRig(), bitmaps, renderTarget, isMultiview);
          returnScreenshot3DToCaller(mScreenshot3DCallback, bitmaps, mReadbackBufferWidth, mReadbackBufferHeight);
  
          mScreenshot3DCallback = null;
      }
docs: capture 3D screenshot