CrossRoads:
For sure there are only 2 External Interrupts.
But almost every pin also supports PCINT, whichNick Gammon has a great page on and even shows them as being a little faster:
Gammon Forum : Electronics : Microprocessors : Interrupts
Nice link, thanks. Just nit-picking, but I believe that Timer1 Capture is also an external interrupt pin, making for a total of 3.