You still haven't told us what you wanna do with your read values. Do they have to be stored in an array? Do you wanna use them directly? What are the three leading characters for? There may be better solutions than to use the fscanf function but without knowledge about your project we cannot suggest you anything.
Given your input and your code, it will never match, as "aaa:" is not at the beginning of the input stream. fscanf does not search for your string, it does try to match a given pattern, returning 0 if this wasn't possible.
I'm so sorry , the fscanf is not the solution to my problem.
I need to read the value of "aaa" from the FILE* var writed by HTTPClient library
Now, i realize that strstr() C function is my best friend so i checked the Arduino String library.
So, if the result of HTTPClient was a stirng i can do this:
String myString = "fff: 10,bbb: 13,ccc: 1,aaa: 15,ddd: 45,";
int first = myString.indexOf('aaa:');
int last = myString.indexOf(',', first);
int result = atoi(myString.substring(first,last));