Under the iPhone source in iTunes, you can choose from a list of various things to sync with that computer's iTunes program.
People are reporting that you can select only contacts/calendar on one machine, and only music/video on another machine and it's working.
But you can't use the iPhone as a middleman to keep two computers in sync with the same items checked on both machines.
Machine #1: music, video, addressbook
Machine #2: calendar, bookmarks
Should work, but:
Machine #1: music, video, ADDRESSBOOK
Machine #2: calendar, ADDRESSBOOK, bookmarks
Would overwrite your addressbook entries.
One computer and one iPhone is simple. But 3+ devices become very complex to sync. If you want to manage 3 devices, you should sync your computers so they mirror each other first (publish calendars to a server, etc), then sync the iPhone.