>This shows all the records from resmngt3 even when resmngt3b has an assignment.
Maybe this will do?
Select *
From webprddt6.resmngt2
Join webprddt6.resmngt3 On webprddt6.resmngt2.recseq = webprddt6.resmngt3.projindex
LEFTJoin webprddt6.resmngt3b On webprddt6.resmngt3.recseq = webprddt6.resmngt3b.taskindex
WHERE assigned = '' OR assigned ISNULL