What are the three guidelines for writing good HTML code?

The  Basic Rules for
Writing HTML

• Use
      well-formed HTML
• Respect
      syntax and semantics
• Don’t
      use presentational or behavioral markup
• Leave
      everything out that is not absolutely necessary
• Standardize
      character case.
• Use
      comments judiciously.

There are lots of rules and recommendations for writing good html.

As someone who has only recently got into writing html and css, the 5 rules that I've found most useful are (I know you said 3 rules, but these are all mistakes that I made when I first started, so thought I'd throw in a few bonus rules):

1. Try to keep your code well-formatted, neat and easily legible.

Opening and closing tags should align, and your html should be structured with indents and spaces in the right places.

2. Don't use html tables for layout

Just don't.

3. Don't use <div> tags unless you actually need them.

This is something I was guilty of initially. Also z-index those divs!

4. Don't use attributes you don't need.

A bit obvious, not every element needs an id, a class, a name and a value... #justsaying

5. If you are going to use a class, id or name - for God's sake make it relevant to what they actually do.

If you're specifying a css id that styles an element to look like a large searchbar with a red border, call it something like #large-red-search

Do not use an id like #qwerty123 because you will struggle to remember what the hell it does when you're going through your code looking for bugs.

Hope this helps!