select distinct sum (case when DC.DocType = 3 then DC.DocID else 0 end) 'ID'
from
	(
	select D.DocID 'Parent'
		, DC.DocID 'Child'
		, DC.DocType
		, count (DC.DocID) over (partition by D.DocID, DC.DocType) Cnt -- Считаем ссылки
	from Docs D
	left join Refs R on D.DocID = R.DocID
	join Docs DC on DC.DocID = R.DocRef and DC.DocType in (2,3)
	where D.DocType = 1
	) X
where X.Cnt = 1 -- Убираем документ-родители с количеством ссылок более 1
group by D.DocID
having (sum (case when DC.DocType = 2 then DC.DocID else 0 end)) * (sum (case when DC.DocType = 3 then DC.DocID else 0 end)) != 0 -- Отсеиваем документы без ссылки на один из типов

CREATE TABLE [dbo].[Docs] 
(
	[DocType] [int] NOT NULL ,
	[DocID] [int] NOT NULL ,
	[DocName] [char] (50) NOT NULL
)
GO

CREATE TABLE [dbo].[Refs] 
(
	[DocID] [int] NOT NULL ,
	[DocRef] [int] NOT NULL ,
)
GO

insert into Docs values(1, 1, 'СТП 2013.001')
insert into Docs values(1, 2, 'СТП 2013.002')
insert into Docs values(1, 3, 'СТП 2013.003')
insert into Docs values(1, 4, 'СТП 2013.004')
insert into Docs values(1, 5, 'СТП 2013.005')
insert into Docs values(1, 7, 'СТП 2013.007')
insert into Docs values(2, 16, 'ГОСТ 2.105-95')
insert into Docs values(2, 17, 'ГОСТ 2.304-81')
insert into Docs values(2, 18, 'ГОСТ 2.307-68')
insert into Docs values(2, 19, 'ГОСТ 3.1109-82')
insert into Docs values(2, 20, 'ГОСТ 3.1118-82')
insert into Docs values(3, 21, 'ГОСТ 7795-70')
insert into Docs values(3, 22, 'ГОСТ 7798-70')
insert into Docs values(3, 23, 'ГОСТ 7817-80')
insert into Docs values(3, 24, 'ГОСТ 15589-70')
insert into Docs values(3, 25, 'ГОСТ 7805-70')
insert into Docs values(4, 26, 'ГОСТ 5915-70')

insert into Refs values(1, 16)
insert into Refs values(1, 21)
insert into Refs values(2, 16)
insert into Refs values(2, 17)
insert into Refs values(2, 22)
insert into Refs values(3, 18)
insert into Refs values(3, 21)
insert into Refs values(3, 23)
insert into Refs values(4, 19)
insert into Refs values(5, 24)
insert into Refs values(6, 22)
insert into Refs values(6, 25)
insert into Refs values(7, 20)
insert into Refs values(7, 25)
insert into Refs values(7, 26)

select distinct sum (case when X.DocType = 3 then X.Child else 0 end) 'ID'
from
	(
	select D.DocID 'Parent'
		, DC.DocID 'Child'
		, DC.DocType
		, count (DC.DocID) over (partition by D.DocID, DC.DocType) Cnt -- Считаем ссылки
	from Docs D
	left join Refs R on D.DocID = R.DocID
	join Docs DC on DC.DocID = R.DocRef and DC.DocType in (2,3)
	where D.DocType = 1
	) X
where X.Cnt = 1 -- Убираем документ-родители с количеством ссылок более 1
group by X.Parent
having (sum (case when X.DocType = 2 then X.Child else 0 end)) * (sum (case when X.DocType = 3 then X.Child else 0 end)) != 0 -- Отсеиваем документы без ссылки на один из типов

create index IX_Refs_DocID_DocRef  on Refs(DocID,DocRef);
create index IX_Docs_DocType_DocID on Docs(DocType,DocID);

select d.DocID
from Docs d
join Refs r on r.DocRef=d.DocID
where d.DocType=3 and exists(
   select NULL
   from Refs f
   join Docs s on s.DocID=f.DocRef and s.DocType in (2,3)
   where f.DocID=r.DocID
   group by f.DocID
   having 
      sum(case when s.DocType=2 then 1 end)=1 and
      sum(case when s.DocType=3 then 1 end)=1
   )

Не по теме: specially для тех, кому CROSS APPLY горло режет, завернул в EXISTS