This is going to be kinda of hard to explain because each country has different grading systems but we are going with the United States's grading system.
And remember, each state is different.
To get an F you need somewhere between 0-64.
To calculate the grade, you subtract the number of questions gotten wrong over the number of total questions from the number of total questions over the number of total questions.
3/3 - 1/3 = 2/3
Which equals approximately to %67 which is pretty much a failing grading.
So, even if you got one question wrong that's what cost it all.