Final answer:
France, Sweden, and the Netherlands intervened militarily in the Holy Roman Empire during the Thirty Years' War on behalf of the Protestants.
Step-by-step explanation:
The three foreign countries that intervened militarily in the Holy Roman Empire during the Thirty Years' War on behalf of the Protestants were:
- France: France joined the war on the side of the Protestants, primarily to become the preeminent power in Europe.
- Sweden: Sweden, a Lutheran land, supported the Protestant German rulers.
- Netherlands: The United Provinces of the Netherlands also supported the Protestant German rulers.