When you say G3, I think you mean the latest Medtronic Guardian 3, which only work with 670 pump.
Before G3, medtronic made sensors called Sof-sensors, then Enlites.
Dexcom originally had a 3 day sensor, then a 7 day sensor they called Seven. Then Dexcom made the G4, then G5. I think the G was for generation. G6 is next.
I started on medtronic Sof-sensors in 2007, then switched to dexcom seven in 2011. Now still using dexcom G4.
Some have posted that the medtronic Guardian3 with 670 is much improved over enlites and close to accuracy of dexcom G4, G5.