If you're wanting to find students that haven't taken any courses for the last 3 years, there's a couple of things I'd change around:
SELECT
stu.campusID AS [Campus ID],
camp.name AS [School],
stu.studentID AS [Student ID],
stu.lName AS [Last Name],
stu.fName AS [First Name]
FROM
students stu
INNER JOIN
campusNames camp
ON
camp.campusID = stu.campusID
WHERE
NOT EXISTS (SELECT * FROM coursesTaken co
where co.StudentID = stu.StudentID and
co.DateTaken > DATEADD(year,-3,GETDATE())
Which hopefully reads more cleanly/closely to your spec. (You can change the DATEADD
back to using 1095 days if that's the actual requirement, but if it's 3 years then the above is better because it takes leap years into account)