EPCOT has by far been DD's (7) favorite park other than MK since she was 2. At age 2, we made the mistake of only doing 1 day there and it was not enough. Since then, we have done 2 EPCOT days every trip. As far as agent P (and its predecessor Kim Possible), I would say 3 is probably too young to get into it. DD got interested in that at age 5. She might enjoy running around watching the 8 year old, and might like the special effects, but as far as actually watching the videos on the communicator and figuring out the clues, I don't think she will have the patience, or be quite up to doing that just yet.
As far as World Showcase- at age 3, DD would have ridden the Donald Duck boat ride inside Mexico in circles all day and been perfectly happy (there is rarely a line for it- but we imposed a 5 time in a row limit!

). She also loved all the entertainers- juggler, bands, balancing act, acrobats, drummers, and stopping to watch them. There are a ton of characters throughout world showcase- Donald, Snow White, Mulan, Belle, Jasmine and Aladdin, Pooh and Tigger, Mary Poppins, Alice, Aurora. DD also liked shopping around WS even at a young age, but then she was born a shopper I think! Surprisingly, at age 3, she loved the movie in China because it was big and all around her. The kid-cot stations are a big hit- currently they have a choice of whether to pick a Duffy or a Perry to decorate. They decorate those with markers. Then, they can take them to the Kid-Cot stations in each other country and they will get a stamp on them from that country. The person at the Kid-Cot station is from that country and they sometimes will teach them a word or two or expression from the country- and might even hand write something for them. Instead of stamping the back, you can also buy a passport ($10 I think), and the kids can take that to each country for stamps etc. There is a small bush maze in the UK that DD loved running through when she was small.