Final answer:
The 3rd drug of choice in the treatment of MI is a calcium channel blocker or a beta-blocker.
Step-by-step explanation:
The 3rd drug of choice in the treatment of myocardial infarction (MI) is a **calcium channel blocker** or a **beta-blocker**.
Calcium channel blockers, such as **diltiazem** or **verapamil**, work by reducing the strength of contraction and relaxing blood vessels. This helps improve blood flow to the heart.
Beta-blockers, such as **metoprolol** or **atenolol**, block the effects of adrenaline on the heart, reducing heart rate and blood pressure, and improving blood flow to the heart.