The - end of the charged battery is already connected to the chassis, the engine, and, particularly, the starter motor. The whole car is designed to work that way. Everything use earth return. All the lights have one wire, and one connection to the chassis. The spark plugs have one wire, and one connection to the engine block. And so on.
OK, on a modern car you use a second wire for the starter motor, because it draws a lot of current and you would get a significant voltage drop and power dissipation in the earth return path. And diesel engines don't have spark plugs. And high power driving lights are driven through a relay like the starter motor is. And lights mounted in plastic fittings need 2 wires.
Given that the -ve end of the battery is connected to the chassis and the shell and the engine, where is the best place to connect the return wire?
Traditionally, you connected it to the chassis or the shell or the engine, so that you didn't have to lean, with a live wire, into the engine compartment, across the engine (hot moving machinery), on to the battery (hydrogen explosion risk, with sulphuric acid as well). But this assumed that the battery was difficult to reach, and that the engine and chassis had a very good connection to the battery (required for the starter motor earth return).
Nowadays, some people make the return connection direct to the battery, if they can reach it easily.
Even if you connect one end of the return wire to the chassis, you normally connect the first end direct to the first battery. This because until the wire is connected to the first battery, it is not a live wire, will not spark when you touch it to something, is not particularly dangerous. After the return wire is connected to the first battery, it is live, and is dangerous. So you connect it to something safe that is easy to reach.
No, you can not connect the return wire to a third car. Cars sit on rubber tires, and the tires insulate each car from other cars. You need a complete circuit from your battery to the other battery.