Does Lumix G3 or GX1 also have shift-sensor image stabilization?

No IBIS (in body IS) on Panasonic m43 cameras. They rely on optical IS but only available on their zoom lenses, not their primes. Oly cameras have the added benefit of turning any prime into a stabilized lens so to speak. With the exception of the EM-5 however the sensor shift IS doesn't work well on video, so in general Panny cameras are better for that, or couple an Oly camera with a Panny lens.