Three weeks into vaping is a common time for a person to start noticing things like sore throat, mouth sores/bleeding gums, excessive phlegm, etc. as the body starts adapting to the lack of the chemicals that are put in cigarettes. It could also be a virus or something...but it shouldn't alarm you too much. It's pretty common to get some uncomfortable symptom or other in the first month or two of quitting smokes.
As everyone has said...stay hydrated. If you don't like water, then even using some sort of flavoring in it (a splash or two of lemon or cranberry juice works nicely) can help. Coffee and tea don't really "dehydrate" you...but the caffeine can complicte things...if you can avoid the caffeinated beverages at least some of the time it's good. Also sodas have a lot of sodium, which is dehydrating. Bear that in mind! The Gatorade type drinks also have a lot of sodium. Flavoring your water and alternating with more favored drinks throughout the day is a good idea.
Me? I love water and drink it all day! Rarely drink a soda or iced tea, but do have about 2 cups of coffee a day in the morning!