The oldest, a 1,1 Mac Pro workstation is just being retired as I have ordered an all singing and dancing i7/Fusion Drive 27" machine today to replace it which I will run as a dual display system with my other 27" Dell monitor.
The new one will have ML loaded from the factory.
The other two (an Air and a Mini) are both able to run ML (whereas the 1,1 is prohibited from doing so by Apple) but are all still on Lion so that we had the same OS on all three to avoid confusion.
Mavericks is due shortly. Would I need to upgrade the other two to ML before Mavericks, or can I just go from Lion to Mavericks on those and skip ML altogether?