I have just brought a 700 MHz iMac & when I turned it on, as usual it asked for the software restore discs. I put the 1st one in & then put the 2nd one in, but when I was getting ready to put the 3rd disc in, it never asked for it & just booted up in OS 10.2 as normal. What is the 3rd disc for, because I have always used 3 software restore discs when booting up a new machine?