Hard to give an exact answer, because a lot will depend on your budget and how keen you are to "travel".
If you have 3 weeks, I suggest that you can take the first week to explore Anilao in Batangas.
There are several islands in that region that you can explore during that time.
The second week, you can proceed to Boracay and explore its surrounding regions. That will leave the third week free for you to explore Manila and its surrounds when you return from Boracay.
