>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 
		LEFT Join webprddt6.resmngt3b On webprddt6.resmngt3.recseq = webprddt6.resmngt3b.taskindex 
	 		WHERE assigned = '' OR assigned IS NULL