30 amps
Yes, three 10 AWG wires can handle 90 amps when wired together in parallel. Each 10 AWG wire can typically handle up to 30 amps, so three wires can handle a total of 90 amps. It's important to ensure that the wires are securely connected and properly sized for the current being carried.
In the United States this is a violation of code. Of course you can physically do it. But the code does not allow parallel conductors any smaller than 1/0 unless each individual conductor is large enough to handle the entire circuit. Using 14 gauge wire you would have to have a circuit no larger than 15 amps, regardless of how many wires you parallel.
50 amps
The current (amp) capability is determined by the wire size in gauge, rather than the voltage rating stamped on the insulation. The most basic rating is 20 Amps for 12 gauge and 15 Amps for 14 gauge wire; however, it can get more complex than that for long runs of wire or for wires run through conduit with other wires. Hire an electrician if this sounds bewildering.
The amps that a four gauge wire will handle will depend with the thickness of the wire. If the wire is thin, the four gauge will handle 95 amps.
Heavier wires do not necessarily mean better sound. Typically, 18 or 16 guage wire is sufficient for home audio systems. If you are going to be running extremely long distances, say 100 feet or more, then increase the gauge size to like 14 or 12 to keep from losing too much audio signal. In a car, 18 guage wire should be quite sufficient.
140 AMPS
The amp size of a wire is determined by its gauge, not the number of wires. An 8-gauge wire is typically rated for around 40-60 amps, depending on the type and insulation of the wire.
15 amps
The recommended wire gauge for carrying 80 amps of current is typically 6 AWG (American Wire Gauge).
An 18 gauge wire can typically handle around 16 amps of electrical current.
An 18 gauge wire can typically handle around 16 amps of electrical current.