Electric heat, I believe, needs more airflow than gas. Flexible duct is an airflow killer.
Add more return and more supply. I agree with SBK, to a degree, by adding just return air doesn't always make a room more comfortable, but, if a system is short on RA, it does effect to supply side. But you're short on both supply and return. Forget the 6" materials, that's a waste of time, money and energy!
Your house must be very nice (and large) to have 3 systems. Were the 3 systems installed when the home was built? I would love to visit GA.