Thank you for your answer. I am confusing about the below line
PWMC_ConfigureClocks( pwm_clockA_freq, pwm_clockB_freq, VARIANT_MCK );
There are only two clock sources A and B, it will affect to all PWM channels (all channels will inherit that two frequencies). So I can not find a way to create the third PWM channel that has a different frequency with A and B. So how I can solve this problem. Thank you so much!