<p>If I put down three interests as a major, do I need to write about all 3 of them in the short answers? Because I would like to focus on only my primary choice.</p>
<p>I think you should focus on your primary choice. It shows your dedication.</p>
<p>I suggest that you write a bit about the others just to show them as to why you choose then but do focus on your primary choice.</p>