How to correctly increment the list numbers for headers in Word

Question

I'm not sure how to get the numbers of my header/title lists to increment correctly in ms Word

I got a document where the first two header levels increment, but I'm trying to create a third level and don't know how to base it on the previous two.

So I want the third level header to look like

5 Title 1
5.5 Title
5.5.5 subtitle

and then bind that to the header.
Can I do this, or do I need to increment the first two values of the list manually?