What I need to change in this code that the names will change till 999 not till 99?
Isn't it obvious?
char filename = "LOGGER00.CSV";
The code is changing the 0 in this string to other values. You need to put a 3rd 0 in the name, without making it any longer.
filename = i/10 + '0';
Replaces the first 0 with the tens value.
filename = i%10 + '0';
Replaces the second 0 with the ones value.
You need to add another statement to replace a position with the hundreds value, change how the tens value is extracted ((i/10)%10) - 864 divided by 10 is 86; 86 modulo 10 is 6), and change the 10 to 100 in the ones statement.