The Gocycle G3 and G3C have the same specs except the material used for the wheels and frame.
The G3C is the carbon fibre edition of the G3. This means the main frame and wheel are constructed from carbon fibre instead of magnesium which results in a lighter weight product.
The Gocycle G3+ is the update of the G3: G3 with carbon wheels and folding handlebar (like the GX). The G3+ is not compatible with the docking station.