I haven't added my site in google webmaster but when I google search my site first 3 results belong to...?

[Tommy]

Hi,

Google probably indexed your site already but that doesn't mean they indexed "all" of your pages. They might not be able to crawl all of your pages and for that reason, submitting a Sitemap to Google Webmaster will allow Google to know all your pages.

Furthermore, since your site is already indexed or crawled, if you search specifically for your site, it should show up since your site matches exactly what you are searching for, your website. What you did is a good way to check if Google still has your site in their index.

However, in SEO point of view, it doesn't mean you did some amazing SEO to your site. You want your site to appear when someone searches for keywords or terms related to your site and not when they search specifically for your site. If they search specifically for your site, it means that the searchers already know about your site. However, for SEO helps people that don't know about you discover your site.

Hope this helps