I have general question, How these three Threads be scheduled in the fashion of multithreading ?

Thread 1 takes a 3 seconds but only use 30% of CPU time.
Thread 2 takes a 3 seconds but only use 30% of CPU time.
Thread 3 takes a 3 seconds but only use 30% of CPU time.

 

You are missing a lot of detailed information.
What is the MCU?
What is the software development platform you are using?
What RTOS are you using, if any?

 

You are missing a lot of detailed information.
What is the MCU?
What is the software development platform you are using?
What RTOS are you using, if any?

I don't have any specific point to your three questions, you can choose as per your wish.

I will assume for example Cortex M3 Linux Operating System and Free RTOS