Join select queries on an id in SQL Server
If I understand correctly, I think this should work...

We're checking that TableA.id+100 is in TableC.eid and then counting the number of times TableA.id+100 appears as TableB.char.

It's always good to create a SQLFiddle though so that others can easily pick up your schema and code to debug.

SELECT
  a.id,
  a.name,
  a.comment,
  COUNT(b.char) AS count
FROM TableA a
INNER JOIN TableB b 
  ON a.id+100 = b.char
INNER JOIN TableC 
  ON a.id+100 = c.eid
GROUP BY a.id, a.name, a.comment;


As Aaron Bertrand pointed out below, using EXISTS would help to eliminate double counting on where duplicate eids appear in  TableC.

SELECT
      a.id,
      a.name,
      a.comment,
      COUNT(b.char) AS count
FROM TableA a
INNER JOIN TableB b 
  ON a.id+100 = b.char
WHERE EXISTS (SELECT * FROM TableC where eid = a.id+100)
GROUP BY a.id, a.name, a.comment;