creating single page with 2 columns while theme is 3 column

You should create a separate template for the home page and use that. Please refer to the WordPress Codex Entry on templates, as it will go more in depth than I can in this answer.

You can then code the template however you would like. You can also style it however you’d like as it will add a different class to the body tag so you can target only that page. Not that a template can be reused for other pages in the future if need be.

If you don’t want to reuse it consider creating a front-page.php, then changing your front page in settings > reading to the page of your choice.