Use https://www.arduino.cc/reference/en/language/functions/communication/serial/readbytesuntil/ to read one line. I know that it's under Serial, but will work for files as well.
After that, you can parse with e.g. https://linux.die.net/man/3/strtok.
- Robin's Serial Input Basics - updated has an example; you can work directly on the buffer where you store the line, you don't need to make a copy.
- Other example: C library - strtok() function