try:

INSERT INTO UNRESOLVED (CaseNbr)

SELECT

 T3.CaseNbr

FROM

 Table3 T3

 LEFT OUTER JOIN 

 (Select CaseNbr From Table 1

  UNION

  Select CaseNbr from Table 2

&nbspQ

 ON T3.CaseNbr =Q.CaseNbr

WHERE

 Q.CaseNbr is Null