You will never manage 3kHz with a 10mS timer.
The period time for 3 kHz is 1/3kHz = 0.333333 mS.
The highest frequency with a 10 mS timer is 1/10mS = 100 Hz.
Actually it is. It is PWM. If you learn how to do one you will know how to do the other.
But as we say in the States. "You are trying to get blood out of a stone". Meaning you are trying to do the impossible. As others pointed out, you need a much faster timer. Did you mean 10u seconds?
You can do it with a timer but you would need a faster timer.
It is actually easier to do it with PWM. Check the top of this subforum for a very nice post on how to do PWM. It is a sticky post.