I guess that the other thing to explain is why we are doing this. I have recently done some user testing on one of the sites I manage and found that none of the users saw the 2nd level of horizontal navigation we have. I know that this second level is used but my feeling is it is used by people who know our site, although I am yet to test this.
The second reason for looking at this multiple level navigation is that if you focus on your user, under your product or services menu, you should have various methods of a visitor finding the content they are looking for. What do I mean? Our products are enclosures, metal boxes basically, people may want to find them by material, application, size, industry etc. Using 3 level navigation you can easily push them to the content they want and reduce the number of steps they have to take to get to their content.
Having said that I intend to test this navigation on our site and see if or user like it. I will report back then.