Try something like this:
from dept in Departments
JOIN course in courses ON dept.CoursesFK equals course.ID
JOIN deptDetail in DepartmentDetails ON dept.DepartmentDetailsFK equals deptDetail.ID
select NEW
{
dept.CommonName,
dept.CoursesFK,
dept.DepartmentDetailsFK,
dept.ID,
dept.LegalName,
course.LegalName as Courses,
deptDetail.Courseyear as DepartmentID
}