Arduino SSCANF stop on comma delimiter
I would suggest using strtok() to chop the string up:

char *prefix = strtok(mystring, ",");
char *firstString = strtok(NULL, ",");
char *secondString = strtok(NULL, ",");


That has the effect of slicing the existing string up in memory replacing the commas with \0 and placing three pointers at the start of the three substrings. It has the advantage that it doesn't need to copy the contents of the string anywhere, so doesn't use any extra memory (other than the few bytes for the pointers).