There was no Mac III because "III" was considered bad luck at Apple for many years following the Apple III's failure.

No 68K Mac series began with "3". We had the 200s (Classics), 400s (LCs), 500s (AIOs), 600s (various), 700s (one Quadra model), 800s (performance), and 900s (performance). In the laptop line, there were 100s, 200s, and 500s--but no 300s.

The rules were finally broken with the LC III (only other Apple to use the Roman numeral) and PowerBook 3400.

Even now, there is officially no "iPad 3". Perhaps a nod to history?