83 - 911SC Coupe—I was also surprised to see this setup when working on my car yesterday! After doing some heavy research and calling mechanic friends, they said these systems are essential to many cars.
The expansion tank (#3) is there if your car ever flips over and gas drains into the "expansion" tank. It's more of a safety tank than a true expansion tank.
Also, hose 6 was unattached and vented in my driver-side rear fender, which is a much better place for your gas to vent out to. You DO NOT want those fumes going back into your car. Before, most cars also had this venting into the rear fender.
If you haven't replaced all the hoses in a while, do so now. They get easily clogged. Imagine that going back into your car. Nope, vent into the fender. Happy Driving!!