There 350, 420 and 504 people work at first, second and third department respectively.
Explanation
Suppose, the number of employees in the first department is

The second department employs 70 people more than the first one. So, the number of employees in the second department

The third department employs 84 more workers than the second one. So, the number of employees in the third department

Given that, total 1274 people work at three departments. So the equation will be .......

So, the number of people in first department is 350, in second department = (350+70) = 420 and in third department = (350+154) = 504