select ASSIGNED_TO,ANSWER = ROUND(CAST(AVG(CAST(Q.ANSWER AS DEC))AS NVARCHAR),2)
from  [dbo].[SCRELATIONM1] S
INNER JOIN  [dbo].[REQUESTM1] R  ON R.[NUMBER]  = S.[DEPEND]
LEFT JOIN [dbo].[QUESTIONNAIREM1] Q  ON R.[NUMBER] = Q.[OBJECT_ID]
WHERE ASSIGNED_TO IS NOT NULL AND ANSWER IS NOT NULL
AND (SUBMIT_DATE  >=  (@StartDate) and SUBMIT_DATE < (@EndDate + 1))
GROUP BY ASSIGNED_TO

select ASSIGNEE,ANSWER = ROUND(CAST(AVG(CAST(Q.ANSWER AS DEC))AS NVARCHAR),2)
from  [dbo].[QUESTIONNAIREM1] Q 
INNER JOIN  [dbo].[INCIDENTSM1] IM  ON IM.[INCIDENT_ID]  = Q.[OBJECT_ID]
WHERE ASSIGNEE IS NOT NULL AND ANSWER IS NOT NULL
AND (OPEN_TIME  >=  (@StartDate) and OPEN_TIME < (@EndDate + 1))
GROUP BY ASSIGNEE

select ASSIGNEE_NAME,ANSWER = ROUND(CAST(AVG(CAST(Q.ANSWER AS DEC))AS NVARCHAR),2)
from  [dbo].[SCRELATIONM1] S
INNER JOIN  [dbo].[PROBSUMMARYM1] P  ON P.[NUMBER]  = S.[DEPEND]
LEFT JOIN [dbo].[QUESTIONNAIREM1] Q  ON P.[NUMBER] = Q.[OBJECT_ID]
WHERE ASSIGNEE_NAME IS NOT NULL AND ANSWER IS NOT NULL
AND (OPEN_TIME  >=  (@StartDate) and OPEN_TIME < (@EndDate + 1))
GROUP BY ASSIGNEE_NAME

select ASSIGNEE , sum(...)
  from (
         тут три селекта, объединенные через UNION ALL
       ) v

SELECT sum_table.person, AVG(sum_table.answer) avg_answer
FROM (
    SELECT person, answer
    FROM tableset1
  UNION ALL
    SELECT person, answer
    FROM tableset2
  UNION ALL
    SELECT person, answer
    FROM tableset3
) sum_table

GROUP BY sum_table.person