how to select multiple items using the AND operator
Your query as it is will always return 0 rows . PROF field cannot be equal to 3 different values at the same time . 
You need something similar to 

select dno 
from employe 
where prof in ('Ingenieur','Technicien','Vendeur') 
group by dno
having count(distinct prof) =3 ;


There are couple more ways to get what you want, you may try them and find which one has better performance .