Hello, Elisabeth!
Great question!
I've traveled to Walt Disney World in mid-to-late March and very early April for the past 6 years, and it's always been warm enough to swim. :-) The pools are heated very nicely, too, so you'll probably find yourselves wanting a dip at least once or twice.
With a 3-year-old, I'd say to stay closer to Magic Kingdom, if you can, especially if you can book a stay at a resort which is on the monorail line. If your little one falls asleep in the stroller, you can simply wheel the stroller right onto the monorail (or even walk "home" if you are staying at Bay Lake Tower or Disney's Contemporary Resort) and tuck the little one right into bed, with little disturbance. March will also offer the opportunity to enjoy
Epcot International Flower & Garden Festival, which is a really wonderful experience for Guests of all ages.
On the other hand, Halloween season at WDW hold not-too-scary fun, too! Even if you don't purchase a
ticket to Mickey's Not-So-Scary Halloween Party, Magic Kingdom's fall finery will provide you with picture-perfect photo opportunities.
For me? That ease of the monorail for any family with a stroller is a huge plus if you can book a stay at one of the Disney Resorts along the monorail line.
Hope that helps! Send more questions as you think of them; we love to help!