Get ancestor at a specific level?
entry.ancestors returns an ElementCriteriaModel (→ getAncestors() in the Craft docs) prepped to return the entry's ancestors.

entry.ancestors.level(3) makes it return only entry ancestors which are at 3rd level of the structure.

As you well pointed out in the comments, an entry can only have one ancestor on a particular structure level. So you can now use the first() method to get this one entry. Keep in mind that if there's no ancestor at that level, the method won't return anything (null).

{% set ancestorL3 = entry.ancestors.level(3).first() %}

{% if ancestorL3 %}
    {{ ancestorL3.title }}
{% endif %}