You can use JOIN to get the data. I have used few fields in the select you can specify what ever you need in the select list
select
u.username,
t.title,
t.Thread,
t.catagory
from
post p
join user u on u.username_id = p.user_id
join Threads t on t.who_id = p.Threads_id
where t.catagory = 'some specific category'