Вот несколько советов, если вы не хотите использовать лимит
select t2.user_id
from (
select row_number() over (order by t1.ct desc) as rn, t1.userid
from (
SELECT user_id, COUNT(user_id) as ct
FROM user_interest
GROUP BY user_id
HAVING COUNT(user_id)
)t1
) as t2 where t2.rn < 3