It's my belief that it may be best to install all 3 mounts at the same time so as to ensure a balanced restraint across the whole engine and transmission assembly at all of the connection points. It may be possible to increase the loading on the remaining oe parts if only one is done. Something to consider.
I agree. I just don't want the extra noise and vibrations since it's my daily driver when my wife needs the GT Sport. When the OEM mounts are dead (which will happen sooner than normal) I can just replace them for the same parts. 
Another agreement here! Doing the dogbone on it's own really punishes the LH gearbox mount and movement from that one feels more noticeable with a beefed up dogbone. The driver's side is quite robust and seems to stand up to it. It's also the LH gearbox one that transmits most of the noise and vibration. Gearboxes are rough, noisy things.
What the car needs is a 4th mount at the front, connected to a cross member. That's what the MK1 had and the engines in that car never flapped about like they do from MK2 onwards. Some cars use hydraulic dampers (like mini struts) to help but I've never seen them on German cars.
