Congrats!!! And it sounds like you have a solid plan to get where you want to be
Yes you can DEFINITELY still lose weight in Phase 3, depending on what your weight is when you enter it. Calorie-wise, Phase 3 is right around (or a little less) than the calories allowed on Weight Watchers - so if they can lose weight on that number of calories, it stands to reason that we would too
The reason some may not lose in P3 has more to do with when they enter it - at a lower weight, your body needs less calories to function, so the difference between P3 calories and what your body needs is going to be smaller. Smaller difference, means little or no loss.