Tidiest way to reduce number of posts on homepage

I am an editor for Technology Bloggers. I want the site to be fast and always try to do things in the tidiest way possible, so that they isn’t loads of messy organised code that takes ages to load.

I want the first page of the site (the homepage) to show the 3 latest posts. All other archives and pages can show 10 posts. The problem is WordPress only lets you choose how many posts to show on a page in general, you can’t (as of yet) say 3 posts on the homepage, and 0 on other pages.

What is the best way to do this? I worry a plugin would slow things down, so am keen if possible to add a line of code somewhere to get everything working.

The site uses a personalised version of the Gravit theme.

I understand HTML well, but only have a very basic knowledge of PHP.