If you cross join your Week and Student tables to get all combinations, and then use not exists to determine where no TimeSheet record exists.
select W.WeekID, S.StudentID
from [Week] W
cross join Student S
where not exists (select 1 from TimeSheet T where T.StudentID = S.StudentID and T.WeekID = W.WeekID);
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solved I have 3 table missing data find in SQL