You might post on the heating and air conditioning forum.

Advantages are that not only is your current equipment sized for the square footage it conditions, but also its ductwork is designed and configured for that specific system with its fan power.

A second system will be less costly and simpler to install. It will offer some "zoning" since it will have its own thermostat control.

And by code, a garage proper cannot have a mixed air system with residential space, due to needed fire breaks and concerns about carbon monoxide if there is a car in the garage. So a separate system would allow the re-conversion of the garage to living space BACK to a garage in the future.

Lastly, in residential HVAC generally 3000 square feet is too big for 1 system.

I can see no advantage to replacing the current furnace and changing its ductwork to tie into the new space. A separate system is the way to go.

Thanks!!! I will also post it in the heating and cooling forum.