I'm going to go ahead and put in a good word for anything that appeals to you as an adult. No matter how many people on this site swear up and down that their toddler's Disney trip was a life-changing experience, I can pretty much promise you that a 3 year old is not going to get that much out of any Disney-specific experience. Sure, they'll enjoy the bright colors and fun activities going on around them, and they may know some of the characters, but it is very likely that they will remember very little of the trip. They could care less that they were at the Poly volcano pool as opposed to any other pool on property; they will enjoy any pool you decide to take them to.
My advice would be to book for convenience knowing you have a toddler, if one of the monorail or Epcot resorts is in the budget. Otherwise, base your decision on what the adults will enjoy. My son was 1 month shy of his 3rd birthday when we went, and he enjoyed the buses, the BWI pools and the Mickey ice cream bar the most.