I have a light controlled by any of 3 switches (3-way/4-way/3-way).

1. Can I replace just one of the 3-ways with a motion sensing 3 way?

2. If the answer to no. 1 is 'yes', IF THE LIGHT WAS TURNED ON BY ONE OF THE OTHER NORMAL SWITCHES, will the motion sensing switch's automatic time-out turn the light OFF after the prescribed time?

Yes, you can replace 1 of the 3 ways with your 3 way motion.
Operation depends on your particular model.