Hi Janette!
Yay for your first trip to Disney! And what a fun age for your kids to experience it!
Having one day for each park is a great plan. There's no need to park hop with three little kids. Plus, there is so much to experience in each park you honestly can't do it all anyway!
BUT don't worry---you'll have such full fun days at the park, you won't be disappointed! My suggestion is to look through the website and the maps to see which attractions are "must-sees" for you and your family. Plan your days around those attractions and all the extra stuff you get to do will be like the icing on top!
Part of the wonderfulness of Disney is just the surprise of it all---so enjoy the walk through the parks, the gift shops and the street shows. Don't get caught up in seeing it all---just enjoy every second you're there!
Also, my favorite thing for kids your ages is the
Magical Beginnings Map. It shows some great attractions for little guys. If you have more specific questions please ask again!