select d.DepName
  from tDep d
 where  3  < (select  1 
              from tUser
             where u.DepID = d.DepID ) 

select d.DepName
  from tDep d,
         (select u.DepID           as ID,
                   count(u.UserID) as Cnt
            from tUser u
           groupBy ID ) as cn
where d.DepID = cn.ID
   and cn.Cnt  >  3 

select d.DepName
  from tDep d
 where  3  < (select count( 1 )
              from tUser u
             where u.DepID = d.DepID ) 

select d.DepName, 
       count(u.UserID) as cnt
  from tDep   d,
       tUser  u
 where u.DepID = d.DepID
 group by DepName
 having cnt >  3 

with
   d (DepId,cnt) as (
      select count(*),DepID from tUser group by DepID having count(*)> 3 
   )
select
   tDep.DepName, d.cnt
from
   d,tDep
where
   d.DepId=t.DepId