Try this
CREATE TABLE #Requests (idReq INT, Descr VARCHAR(50))
CREATE TABLE #RequestProcess (idReq INT, idProc INT)
CREATE TABLE #Processes (idProc INT, Description VARCHAR(50))
INSERT INTO #Requests
SELECT 150, ' Test A'UNION
SELECT 151, ' Test B'UNION
SELECT 152, ' Test C'
INSERT INTO #RequestProcess
SELECT 150, 1 UNION
SELECT 150, 2 UNION
SELECT 150, 3 UNION
SELECT 151, 3 UNION
SELECT 152, 1 UNION
SELECT 152, 3
INSERT INTO #Processes
SELECT 1, 'A' UNION
SELECT 2, 'B' UNION
SELECT 3, 'C'
SELECT idReq,
SUBSTRING(
(SELECT ', ' + Description FROM #RequestProcess RP2
INNER JOIN #Processes P ON RP2.idProc = P.idProc
WHERE RP1.idReq = RP2.idReq
FOR XML PATH('')), 3,1000) AS CsvProc
FROM #RequestProcess RP1
GROUP BY IdReq
DROP TABLE #Requests
DROP TABLE #RequestProcess
DROP TABLE #Processes