Emma,
Using 7 seg displays is cumbersome way to tie up an Arduino.....IMHO...An LCD would be better and could display a bit more info too....
Let's see, 3 led displays, 7 cathodes and three anodes, 10 pins; LCD display 4 data lines, reset and enable...6?
A suitable driver chip could reduce the Arduino pin count...depends on what you want....
I'm sure I've seen code snippets on the Arduino site, saves having to write your own....
Ddoyle, I'd be interested to see how you do yours....

Are you using an Arduino or something else?