Get the three last Matrix blocks I would like to include a file on every block but the last three. How can I accomplish this?
You can fetch all elements and slice the last 3 elements from them

{% set entriesList = entry.fieldHandle.find() %}
{% set start = entriesList|length - 3 %}
{% set end = entriesList|length %}
{% set lastThree = entriesList | slice(start, end) %}


Of course you need to make sure you have at least 3 objects your array