The deaths of two boys and the girl were reported from Hubei and Jiangsu provinces before the Gaokao, the Chinese name for the exam.
Two boys committed suicide while the death of the girl is still being probed.
The reports about the deaths were released by police after the exams on Tuesday so that they will not have any impact on the general mass of the students appearing for the tests, local newspaper Global Times reported.
Almost every year, reports emerge in Chinese papers of students committing suicide before the entrance exam.
Some have histories of metal instability, but others just seemingly collapse under the weight of the pressure to do well, as their future careers depend heavily on their score, it said.
The Chinese Gaokao (pronounced gow kow) is like the American SAT or ACT tests, in terms of its importance in getting into a university.
However, it's about a third longer, at least nine hours, and it is completed over two days in most localities.
It is also offered just once a year, and it is the only college-entrance exam for all Chinese universities.
This year marks the 33rd year of the Gaokao, which was restored in 1977 as China began opening its doors wider to the outside world.
An estimated 9.57 million people sat for the exam this year.
"China may be changing at head-twirling speed, but the ritual of the Gaokao remains as immutable as chopsticks," the New York Times commented in a report a year ago.
Peter Foster, a China correspondent for the Daily Telegraph, commented that the dreaded Gaokao is a marathon exam with "questions to make you quail".
"With so much at stake - there are far fewer soft landings in China for those who don't make the cut - it's not surprising that these exams bring out stresses and strains on students and parents alike," he wrote last year.
In each city, police officers used their police cars to pick up students, and vehicle sirens were forbidden as students sat for exams.
Parents waited for their children outside the gates of schools, sometimes joined by grandmothers and grandfathers.
A survey jointly conducted by Sina.com and consulting company MyCOS, released in May, showed that 75 per cent of senior high school students felt that they were under great pressure from February to April, and more than 63 per cent of parents were also stressed out.
The conclusion was based on 14,892 student questionnaires and 16,507 parent questionnaires received from 16 provinces and cities.
Well-known education expert Yu Minhong suggested at a forum in April that the exam should be held twice each year, and students could contact schools based on their best grade from the two exams, thereby easing some of the pressure on them.
In fact, the exam fever in the past few years has appeared to be waning, as unemployment has become a growing concern for university graduates and more students apply to study abroad or start up their own business.
Statistics from the Ministry of Education show that the number of examinees has fallen for two years, with this year's number 650,000 fewer than last year's 10.2 million.
Han Han, a 27-year-old well-known writer in Shanghai, has long been seen as a successful high school dropout.
Han's first novel, published in 1999 and based on his experience of quitting school, became a best-seller in China and sparked a debate about the quality of the country's rigid education system, Time magazine commented in April.
Geng Shen, a researcher with the Beijing Academy of Educational Sciences, said the root cause for the frequent deaths is not the Gaokao itself, which he called the farest exam in China.
"Society should be blamed for the deaths, as the evaluation of a person's worth is wrong. Some professions and professionals are looked down upon by society, and parents want their children to become public servants, the rich or celebrities, which will win respect from society," he said.