Tbl_User (UserId, Username)
Tbl_Customer (CustomeriD, CustomerName)
Tbl_DocA (DocId, CustomerID, DateCreate, DateAdd, UseriD)
Tbl_DocB (DocId, CustomerID, DateCreate, DateAdd, UseriD)
Tbl_DocC (DocId, CustomerID, DateCreate, DateAdd, UseriD)

DECLARE @UserId int = 1; -- or whatever the id of the user you need
WITH CTEDocA AS
(
    SELECT CustomerID
         , COUNT(DocId) As NumberOfReports
         , COUNT(CASE WHEN UserId = @UserId THEN 1 END) As NumberOfReportsByUserAzerty
    FROM Tbl_DocA
    GROUP BY CustomerID
), CTEDocB AS
(
    SELECT CustomerID
         , COUNT(DocId) As NumberOfReports
         , COUNT(CASE WHEN UserId = @UserId THEN 1 END) As NumberOfReportsByUserAzerty
    FROM Tbl_DocB
    GROUP BY CustomerID
), CTEDocC AS
(
    SELECT CustomerID
         , COUNT(DocId) As NumberOfReports
         , COUNT(CASE WHEN UserId = @UserId THEN 1 END) As NumberOfReportsByUserAzerty
    FROM Tbl_DocC
    GROUP BY CustomerID
)

SELECT cust.CustomeriD
      ,cust.CustomerName
      ,ISNULL(a.NumberOfReports, 0) As NumberOfDocA
      ,ISNULL(a.NumberOfReportsByUserAzerty, 0) As NumberOfDocAByAzerty
      ,ISNULL(b.NumberOfReports, 0) As NumberOfDocB
      ,ISNULL(b.NumberOfReportsByUserAzerty, 0) As NumberOfDocBByAzerty
      ,ISNULL(c.NumberOfReports, 0) As NumberOfDocC
      ,ISNULL(c.NumberOfReportsByUserAzerty, 0) As NumberOfDocCByAzerty
FROM Tbl_Customer cust
LEFT JOIN CTEDocA As a
    ON cust.CustomeriD = a.CustomerID
LEFT JOIN CTEDocA As b
    ON cust.CustomeriD = b.CustomerID
LEFT JOIN CTEDocA As c
    ON cust.CustomeriD = c.CustomerID

select
  customerid,
  count(*),
  count(case when userid = <particular user ID here> then 1 end)
from tbl_doca
group by customerid;

select
  c.customerid,
  c.customername,
  doca.total  as doc_a_total,
  doca.az     as doc_a_by_azerty,
  docb.total  as doc_b_total,
  docb.az     as doc_b_by_azerty,
  docc.total  as doc_c_total,
  docc.az     as doc_c_by_azerty
from tbl_customer c
cross join
(
  select userid from tbl_user where username = 'Azerty'
) azerty
cross apply
(
  select
    count(*) as total,
    count(case when da.userid = azerty.userid then 1 end)n as az
  from tbl_doca da
  where da.customerid = c.customerid
) doca
cross apply
(
  select
    count(*) as total,
    count(case when db.userid = azerty.userid then 1 end)n as az
  from tbl_docb db
  where db.customerid = c.customerid
) docb
cross apply
(
  select
    count(*) as total,
    count(case when dc.userid = azerty.userid then 1 end)n as az
  from tbl_docc dc
  where dc.customerid = c.customerid
) docc
order by c.customerid;