Also, remember that if the LED are rated 10mA (for example), if you drive them 1/3 of the time, it will only light 1/3 of the power.
LED are beautifull thing for that is you can put higher current, if the duty cycle is low. For example, you can run them with 30mA, 1/3 of the time. But make sure though that your system actually does toggle between the digits, and does not get stck at one of the digit.
Note though that there is a limit on the ammount of current you can put in that way, and that the voltage drop may be a bit higher across the LED. Check with the datasheets.