Just an educated guess here. I believe the RSO3 Middle Tier is (or may be) related to RoboSource Control. Either that or it's related to RoboHelp's built in ability to work with other source control systems.
So how did you disable it? I'd like to but as yet have never actually found where I can!
Helpful and Handy Links
Yes you are right - it is a part of RoboSource Control. Still the problem remains - it adds some references to the RSO3 Middle Tier in MS Visual Studio, which shows nasty messages upon loading. I have been checking the docs, but cannot find the place where to switch this creature off.
Finally, I have resolved the issue by removing the RoboSource Control from the system. A "hammer" method...