This profile will let you change the ringtones on every new call you get. I am assuming that we have just three ringtones to change. If you have more, you can make changes accordingly.
Ringtone xxx If %COUNT = 0
Ringtone yyy If %COUNT = 1
Ringtone zzz If %COUNT = 2
Add Var %COUNT, 1
Set Var %COUNT 0 if %COUNT > 2
Run the task on Event: Phone Ringing
p.s. Ringtone is in the Media category