A search in the upper bar (of arcgis.com, not just forums) on "multiple ArcSDE services"
pulled up at least half a dozen likely links in the first page (of 1100+ matches). If you
remember to use the proper product name, the search engine should be reliable.
Keep in mind that you're likely to run into the non-interactive desktop heap limitation
on Windows hosts if you run more than 66-70 total application server clients across
the three instances.
- V