Depending on the diameter of the spoke holes in your current rear hub, which will match a replacement hub only by happy coincidence, you'll need new spokes as well, and you'll need to run your specs of your new set up through any one of a number of online spoke length calculators to figure out the correct replacement spokes length.
The Shimano Nexus 3 disc hub is
expensive; I'd also be considering a
Sturmey Archer 3sp hub @ $40 cheaper. And I'd be weighing the cost and effort of building a wheel/having a wheel built with a new hub vs. buying an
off the shelf built wheel.