Misunderstanding the difference between single-threading and multi-threading programming

Question

I have a misunderstanding of the difference between single-threading and multi-threading programming, so I want an answer to the following question to make everything clear.

Answer 1

As I understand, only ONE thread will be executed at a time

That would be the case if the CPU only had one core. Modern CPUs have multiple cores, and can run multiple threads in parallel.

The program running three threads would run almost three times faster. Even if the tasks are independent, there are still some resources in the computer that has to be shared between the threads, like memory access.