I had done a very nice long post with average costs of different class restaurants, and a whole load of other stuff, but unfortunately the websirte didn't seem to want to accept it. So here is the concise version of that rather long winded ( me surely not

LOL)
With the using lower end of all the different choices I came up with a cost of GBP 1350 (that was using BK/McD's/dunkin donuts) at least 3 times a week as breakfast and dinner options and not allowing for buying some sort of food at lunch in the parks (which would add about GBP 300 to the total)
Mid scale (Denny's/Shoney's/Olive garden) came in at about GBP 2,000 (2,300 if you eat lunch in the parks each day)
Midscale and a couple of splurges gets you close to GBP2,500
As far as "cash needs" obviously almost all of those restaurants will take plastic, but as far as budgetting the expense ( or if you'd rather not have that big a CC bill when you get home) I think those are reasonable estimates to go by.
Much depends on the level you like to eat at, and if you like a beer/glass of wine or two as alcohol is relatively expensive in the USA. That could easily add $20-30 per evening meal ($600-900 for the 3 weeks)
Couple of things to minimise your (soft)drinks bills in the parks. Share a larger drink between 2 (or more) instead of getting a smaller one each and order it with no ice. Ice makes up about 50% of the volume of the drink and it either get's thrown away or waters down the drink until it tastes unpleasant, then gets thrown away. Also taking a drink (or two) each from the supermarket/hotelroom/villa could easily save you $15-20 a day.