29A is the stock fulcrum with no jammer, pretty crappy radar, all the typical A/A and A/G stuff (R-27s, R-73, R-60, 80mm rockets, dumb bombs, cluster bombs, maybe dispensers and illumination bombs, dont' remember)
29S has a better radar (multi target track), internal ECM (inferior to other FC3 birds), R-77 capability, slightly more internal fuel.
29G is a 29A slightly downgraded, limited to R-27Rs only (no T/ET/ER) for BVR. it uses feet/knots for the displays EXCEPT for target range, that's still in KM. Limited to rockets only for air to ground. Don't think there are any differences in the radar besides the fact that target speed/alt are in knots/feet.