Q. What happens if the ball you are playing loses pressure (or pops) in the middle of a point. I know this is rare, by my friend was playing a point and everyone noticed the ball had lost pressure. They played the point to the end however.
A. If a ball breaks during a point (not just a soft ball but a ball that breaks and now has no pressure) then that ball should be replaced and a let should be played, even if it is discovered after the completion of the point. If it is discovered that the ball is soft but not broken, that ball should be replaced however the point stands.
Q. Is there a rule governing use of new balls for a third set?
A. The ball change for matches is determined by the official in charge or the procedures of the event or competition that was announced prior to the matches.