Only wrote 1 sentance for 1/3rd of the exam? Things are not looking bright for you, my friend... whether you've taken a hit for the rest of your classmates by volunteering to suck up an 'F' will depend alot on how well you answered those other 2 questions. Of course, the performance of your classmates will really determine your overall mark, but not all those potential points you forfeited will be hard to overcome.
Regardless, your school's student handbook, which is usually available online, should explain their particular policy on failing grades and retakes...
Best of luck,
Rob
PS: Assuming that this wasn't your last final, you really need to figure out what went wrong. Did you blank on that last question? Run out of time answering the first two? Whatever was the problem, you CANNOT repeat it on your next exam. Don't worry about this one's grade right now... you'll have plenty of time after finals to worry about your grade. For now, just focus on your remaining exams since this one is now beyond your ability to influence it...