Quite a while back I stumbled upon a 3D variant of the game go. What I recall of the rules:
- designed to be playable without extra equipment beyond go board and stones
- played on a 9x9 grid
- stones can be stacked by putting one piece above/between 4 pieces, forming a little pyramid
- capturing stones works within one plane as normal, except that stones under other stones are pinched and can't be captured
The whole ruleset is online somewhere. However my google fu is weak and with the terms go, weiqi, baduk, 3d and pyramid I had no luck. I also found no mention while skimming this reddit or this entry in the sensei's library.