Here is a 180 equirectangular stereo movie done with a twin Gopro 3+ Black righttps://www.dropbox.com/s/gslyccr43c8whwy/eveleighmarkets.mp4
In VR Player under Media you need to set Format to Side by Side and Projection to Dome
Until recently there was no way to get good synchronization reliably with recent Gopros for stereo but there is now a sync cable you can buy from http://3dguy.tv/
which works well. I am not sure if the concept behind it can be extended to work with more than 2 cameras
Basically my rig for this video looks like this http://www.mediavr.com/gopro3plusfisheyestereorig.jpg
with replacement Sunex lenses to get about 180 by 150 degree coverage on the 1920 by 1440 format on the Gopros
It is very simple to shoot 3d for the Rift with this because all you need to do is hold the camera level and point it generally in the right direction.
For more accurate composition I can plug a Gopro LCD Backpak into the cable or I can plug a 7" monitor into the HDMI socket of one or other of the Gopros -- I have this onehttp://www.ebay.com.au/itm/FEELWORLD-7-TFT-LCD-HDMI-Monitor-FW678-HD-O-W-Sun-Shield-Shoe-Mount-f-Camera-/141125167872?pt=LH_DefaultDomain_15&hash=item20dbb72b00&_uhb=1
With two LCD Backpaks connected you can get stereo previewing of the video if you arrange some optics (spare Rift lenses maybe).