Power Supply for Nema 17 motors
Some where in the post I read that if there is a CNC with 3 motors for
  3-axis than at a time only one motor is active.


No, if you want to make diagonal cuts, then two motors need to be active.


  Q.1 Is this true if I run the program with GRBL using Ardunio UNO? And
  need the power supply of one motor for running 3 motors?


Size it for three motors just to be safe. Always size power supplies for maximum load.


  Q.2 How can I calculate the required power supply?


From Pololu:


  This breakout board for TI’s DRV8825 microstepping bipolar stepper
  motor driver features adjustable current limiting, over-current and
  over-temperature protection, and six microstep resolutions (down to
  1/32-step). It operates from 8.2 V to 45 V and can deliver up to
  approximately 1.5 A per phase without a heat sink or forced air flow
  (rated for up to 2.2 A per coil with sufficient additional cooling).


So, you need whatever the voltage your running the motors at, times the current to find the power for one motor.

So if your running at 12V, then \$12V*1.5A = 18W\$

And if you need that for each channel, then you'll need about a 60W supply running at 12V. Thats 12USD for a cheap one.

If you have heatsinks the current is prorated to 2.2A