Writing numbered headings in Writer

Just

3. Hit F11 which opens the styles and apply the styles to heading 1 to 3.
4. Put your cursor in one of the headings and hit Format/Bullets and Numbering... (update: and not Tools/Outline Numbering... as mentioned before) in the menu bar.
5. Then select the tab outline and choose the option numeric with all sublevels (see image)

In the position tab of 'Bullets and Numbering' you can choose... their position of course. If you want the typical indent you suggest, just click default in the three different levels.

Enjoy.

If you follow the answer of @don.joey, you will make the WHOLE YOUR DOCUMENT A NUMBERED LIST! Better avoid it! There are more natural native ways to number the headings. The Tools -> Chapter (Outline) Numbering is a much better way to go!

If you encounter some unexplainable problems sometimes you might need to reset the problematic heading style (change it to Default and back to Heading).