Final answer:
The verb 'hace' should be used to fill in the blank in the sentence.
Step-by-step explanation:
The correct verb to fill in the blank in the sentence 'tres años que hablo español' is hace.
This sentence is expressing the duration of time that the subject has been speaking Spanish, so we use the verb 'hace' to indicate the passage of time.
For example, 'tres años que hace que hablo español' means 'I have been speaking Spanish for three years'.
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