How do I set it up so that the newest machine of the 3 is the "Master" and the other 2 older machine are the "Slaves"?
Dropbox doesn't work that way. There is no master or slave. All systems sync to your account online, and any change made on any system will sync to every other system.
I notice that of the 3 machines, I can only access the "Desktop" of the 2 newer machines only.
Did you enable the computer backup feature for those two systems, but not on the older one? The computer backup feature moves your Desktop, Documents and Downloads folders into the Dropbox folder under a subfolder for that computer. If you haven't done that on the older system, then Dropbox isn't syncing the Desktop for that system.
How do I set it up so that the newest machine of the 3 is the "Master" and the other 2 older machine are the "Slaves"?
Dropbox doesn't work that way. There is no master or slave. All systems sync to your account online, and any change made on any system will sync to every other system.
I notice that of the 3 machines, I can only access the "Desktop" of the 2 newer machines only.
Did you enable the computer backup feature for those two systems, but not on the older one? The computer backup feature moves your Desktop, Documents and Downloads folders into the Dropbox folder under a subfolder for that computer. If you haven't done that on the older system, then Dropbox isn't syncing the Desktop for that system.
