try SQL Dense_Rank() window function instead:

with a1 as (

select d.name as department, e.name as employee, e.salary as salary,

dense_rank() over (partition by d.name order by e.salary desc) as dense_ranked

from employee e join department d on e.departmentId=d.id

)

select department, employee, salary

from a1

where dense_ranked <= 3;