var query =
Accounts.Join(StudenTables,
a => new { a.Alias },
s => new { Alias = s.stdID },
(a, s) => new { a, s }).
Join(Courses,
x => new { CourseCode = x.s.stdID.Substring(2, 3) },
c => new { c.CourseCode },
(x, c) => new { x.a.Alias,
c.CName,
x.s.stdName,
x.s.stdFather,
x.a.AccountId }).ToList();