Don't know that I completely understood the table/data setup but hopefully this at least points you in the right direction

DECLARE @TblA TABLE (

Col1 INT

, Col2 INT

);

DECLARE @TblB TABLE (

Col1 INT

, Col2 INT

);

DECLARE @TblC TABLE (

Col1 INT

, Col2 INT

);

INSERT INTO @TblA (Col1, Col2) VALUES (1, 1),(2, 2);

INSERT INTO @TblC (Col1, Col2) VALUES (100, 1),(200, 2),(300, 3);

INSERT INTO @TblB (Col1, Col2) VALUES (1, 1),(2, 1),(2, 2),(2, 3);

Select A.Col2

, STUFF((

SELECT',' + CONVERT(VARCHAR,c.Col1)

FROM@TblB B

Inner join @TblC C on c.Col2 = b.Col2

WHERE B.Col1 = a.Col2

FOR XML PATH(''),TYPE).value('.','VARCHAR(MAX)'),1,1,'')

From @TblA A