You will definitely have some relaxation time as you are going for 3 weeks. Plan your days 'off'.
My sister recently bought a 14 day Disney ticket from her tour operator which was cheaper than buying a few days when there. This gives you freedom over that 2 weeks to go as you please. Relax during the day and hit the (later opening) parks during the late afternoon/evening.
Check out Seaworld's website while youre here in the UK. I got a fantastic deal from them last year, 14 days access for £10 more than a one day ticket. They also have a UK operator number who give excellent service - dont pay to have special delivery of the tickets, mine arrived the next day first class post.
14 day tickets start from the day you first use them - so you can stagger the starts to cover all 3 weeks, if you decide to go that way.
If Disney feels a bit daunting, pop into Downtown Disney first (free entry and parking). There is a guest services office (near the shop TrenD) which will have maps and times guides for all of the parks and attractions. Take them all and have a look at what you all would like to do first/spend more time etc.
Sorry if this is a bit long, bit of a veteran - been going since 1989 :-)
-At Universal, Mythos is pretty good (flatbread and Pad Thai are great!) as well as 3 Broomsticks.
-I'm not familiar with food at Sea World or Busch
-At Disney, your options are endless. My favorite meals on my most recent trip were at Sanaa, Le Celier, Be Our Guest, and Brown Derby(actually trip before last). All are great, but each have their drawbacks. Le Celier has the best food, but with 9 people, we splurged and spent $800. On a budget (no alcohol, super pricy food, etc..) 4-5 people can probably do it for about $200. Sanaa is pretty cheap with incredible, but unique food that may upset the stomach. It's primarily Indian food. Be Our Guest is slightly more expensive than Sanaa, but the food is just as good (French/American) and the setting is great! However, the wait has the potential to be tremendous even with reservations (We got there 20 min early, but were let in 20 min late...not fun). The Hollywood Brown Derby is slightly more expensive, but the food matches the price. There are plenty of other nice options for food, but don't go into a place without knowing what you are getting yourself into.
so to reiterate, get 2 lots of 14 days, check out the all you can est places, Golden Corral was a family fave last year, the sizzler does an ok job as well. There are plenty of options, even dropping into Macdonalds or a KFC, the 192 is littered with them.
For relaxation, if your villa has a pool then take advantage, use it early evening after you been shopping, oh that reminds me, the Florida Mall is pretty sweet, there are a couple of flea markets in the area and up in Osceola you get the farmers market on a fri/sat/sunday which is good for a visit.
Try to avoid the parks on a weekend would be my big tip, your more than welcome to go of course but experience says its buiser, more crowded and you find yourself wishing you had gone to the beach instead.
Sorry, my advice has turned into a travel guide oops. Whatever, enjoy yourselves and raise a butterbeer to you friends here at TPI when your in the WWoHP.
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood