我理解：question是问题表，罗列所有的提问
answer 是回复表，对某个具体问题的回复，用question_id与question表中的id做关联
reply 是对某个回复的评论，用answer_id与answer表中的id做关联
以下代码，基于以上理解

select t1.q_id as 问题id
, t1.q_user_id as 提问者id
, t1.q_content as 问题内容
, t2.a_id as 回复id
, t2.a_user_id as 回复者id
, t2.a_content as 回复内容
, t3.r_id as 评论id
, t3.r_user_id as 评论者id
, t3.r_content as 评论内容
from 
(
select id as q_id ,user_id as q_user_id ,content as q_content
from question
) t1 -- 所有的问题列表，用id做唯一性的区分
left outer join
(
select id as a_id ,question_id ,user_id as a_user_id ,content as a_content
from answer
) t2
on t1.q_id = t2.question_id -- 每个question_id对应的回复
left  outer join
(
select id as r_id ,answer_id ,user_id as r_user_id ,content as r_content
) t3 
on t2.a_id = t3.answer_id -- 每个answer_id对应的评论

select reply.,answer.,question.* from
reply
right join answer on
reply.answer_id = answer.I'd
right join question on
answer.question_id = question.id
Where question.id =(查找的question.id)