Rob Rosenbaum has a slightly terrifying question.
He’s been syncing photos from a single iPhone to two different Macs via USB—just importing them on each Mac.
Once I turn this on for the first time, will iCloud import everything from all three?
Before you change any settings, I recommend making a full local backup of all your libraries, just in case.
Photos and iCloud Photo Library is much better and more stable than in its earlier releases, but you never know what might happen.
Next, I’d suggest picking one device to start with.