Here are the steps to merge the data files:
1. Open the file Student_1 containing information on the first 5 students.
2. Go to Data > Merge Files > Add Cases.
3. Select the file Student_2 containing information on the additional 3 students.
4. The cases from Student_1 will be merged with the cases from Student_2.
5. Save the merged file as Student.
6. Open the file Student.
7. Go to Data > Merge Files > Add Variables.
8. Select the file Student_3 containing the Marks variable.
9. The variables from Student_3 will be added to the cases in Student.