Agreeing with spiceycat on both points -- SeaWorld is kind of in between the other two, and it's going to be pricey to do all three in one trip! The way WDW ticket prices work, especially, it's pricier to do it that way.
That said, there are a bunch of hotel choices along Westwood Boulevard and maybe even down International Drive to the south, if you're willing to go as far as Floridays and the Fountains and Sheraton Vistana Villages. North of SeaWorld is where International Drive gets really busy pretty quick; traffic's generally better to the south, in my experience.
Also, if you're going at a busy time you might consider a split stay, since staying onsite at Universal can get you early entrance and a front of the line pass.