Is all this happening inside a form screen, or do you just have some input components on a details screen?
Either way, you’ll need to write the entered Roll No. to a user specific column. You can then use the entered value to build a relation to whichever table holds the other two values, and fetch them via lookup columns.
If this is being done inside a native form screen, user specific columns cannot be used and so you would need to switch to a custom form.