They are similar, and in some sentences you can use whatever, but no always.
Pretender, is have an intention or purpose to do something, but it just in your mind, you didn't do anything yet. Pretendo terminar la lección antes del fin de semana / I pretend to finish the lesson before the weekend. You can understand that you didn't start it yet.
Intentar, is have an intention but with effort or enthusiasm. Intento terminar la lección antes del fin de semana / I try to finish the lesson before the weekend. You can understand that you already started and you are working on it.
Tratar, is more similiar to Intentar, you can translate "try" but have differents meanings like, "treat" or "deal with".