WOW three weeks at Disney.....lucky you

!
We've stayed at the All Stars / Riverside / POP on different visits and out of them all I enjoyed POP the most

. Yes you can pay more at moderate and deluxe resorts but how much time do you actually spend in your room. I didn't think the room at Riverside was that much bigger than the value resorts and it was a longer walk to the main complex for food. I would rather stay at a value and have extra money to spend while there.
Look out for a fridge swap

if you go to POP or the Allstars so you can store some food in your room (there is a separate thread for fridge swaps). Handy for breakfast and supper and much cheaper that renting one from Disney specially if you are there for 21 days.
We have never rented a car while their if you staying on Disney then you can use their transport to get around quite easily. DTD

is good to give you a bit variety on the meal front too. There is a lynx bus that goes from just across from DTD to SeaWorld and you can change their to get up Int Drive on the Ihop or another lynx bus. There is also other places at the Crossroad where you can eat and shop too.
The first two times we went to Florida our kids only wanted to go to the Disney parks and we were there for 14 days.
You'll get plenty of tips here

.....the planning is half the fun.
Hope you have a great holiday

.