Three, it is known that an exam consists of 6 questions, each of which is solved by exactly 100 people. And any two participants have at least one question that neither of them solved. How many people at least participated in the exam?
At least 200 people. Number 200 people. Numbers 1～50 solve questions $1$, $2$, $3$, numbers $51 \sim 100$ solve questions 1, 4, 6, numbers 101 150 solve questions 5, 2, 4, numbers 151 200 solve questions 5, 3, 6. At this point, each question is solved by exactly 100 people, and any two people have a question that neither of them has solved. On the other hand, if the number of people $n<200$, then since $3 n<600$, there must be a person $A$ who has solved at least 4 questions. Suppose $A$ solves the first four questions. If $A$ also solves the fifth question, then the person who solves the sixth question has no question that both of them have not solved, so $A$ can only solve 4 questions.
Since $n<200$, there must be a person $B$ among the remaining people who solves the last two questions. At this time, $A$ and $B$ have no question that both of them have not solved. The contradiction shows that the number of people must be $\geqslant 200$.