Final answer:
The appropriate verb to fill in the blank is 'hace.'
Step-by-step explanation:
The appropriate verb to fill in the blank in the sentence 'tres años que hablo español' is 'hace'.
The correct sentence would be 'hace tres años que hablo español' which translates to 'I have been speaking Spanish for three years.' The verb 'hace' indicates the passage of time in this context.
Learn more about Verb conjugation in Spanish