How to set index page as parent of a structure I don't know if it's possible, but what I would like is to have a structure with 3 levels linked to a single entry that would act as the index. The problem is the examples I've seen are only with 1 level structures, so what they do is they set the single index URI as services/ and then the structure URI as services/{slug}. This, for a level 3 structure, gives me services/level3slug. 
I want to display the parent URI on the level 3 entries, but if I do services/{parent.uri}/{slug}, I end up having a URI that is services/level1slug/services/level2slug/services/level3slug. When I would just have services/level1slug/level2slug/level3slug.

Is there a way of setting the single page as parent of the structure so I can just do {parent.uri}/{slug}?
If I understand your question, you want to combine a Single and a Structure into the same family of URL's, something like this:

/services - (Single)

/services/service-01 - (Structure Entry)

/services/services-01/example-sub-service-01 - (Structure Entry)

/services/services-01/example-sub-service-02 - (Structure Entry)

/services/service-02 - (Structure Entry)

/services/service-03 - (Structure Entry)



As you rightly said, you would create a Single for your Services homepage and set the URI to services.

You would then create a Structure called "Servcies", and set the URI to:

{parent.uri ? parent.uri: '/services'}/{slug}

This nice bit of Twig checks if parent.uri is defined, and outputs it if it is. Otherwise (if the entry is top level in the structure, it just outputs /services/{slug}