Hello,
If your ‘marked complete’ fields are checkboxes, you can create formula field in second table, smth like SWITCH(ref1+ref2+ref3,3,‘all returned’,2,‘one pending’,1,‘two pending’,0,‘three pending’) and choose it for lookup added to the link in first table.
Otherwise, check formula field reference to get desired result.
For text values, you can put LEN(CONCATENATE(ref1,ref2,ref)) in switch expression.
I would suggest to get numeric result to avoid multiple nested IFs.