Correct, the "big three" are Cornerstone, Avimark and Impromed. There are already several saas solutions as well, such as Neo and countless small vendors. It's telling that the top three are all now owned by either a lab services company (Idexx owns Cornerstone, DVMax and Neo) or a major supplier (Henry Schein owns both Avimark and Impromed). These products are probably no longer seen as strong businesses on their own but rather they provide a value to the parent company from integrations.
This article is spot on: http://avc.com/2014/07/the-dentist-office-software-story/
The software is almost secondary for IDEXX, though. They're primarily a national chain of veterinary labs -- bloodwork, tissue sample analysis, that sort of thing -- so they've got some pretty huge hooks into the industry.