Yes you can use any G3 equipment with your existing G1/G2 kit.
If using G1 equipment please note that the 'pilot tone' was not a feature... and therefore please ensure 'pilot tone' is turned off on any G2 or G3 equipment.
Some additional features may not be usable i.e. the sync function will not be possible for G1/G2 devices.
You will also find that the pre-programmed banks of frequencies differ between G1, G2 and G3, but you can use the tune function to match one set to the other.