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As you are holding an onwards ticket you will be granted entry into Cuba for 72 hours to allow for you to make your next flight. You will be able to leave Terminal 2 to go outside of the airport and transfer onto Terminal 3. Or, anywhere else in the country during your 72 hour period as long as you are back to take your next flight.
You can enter Cuba, but your transit cannot last more than 72 hours from your hour of arrival. (source)
Also confirmed by Timatic, the database used by airline employees:
Visa required, except for Holders of confirmed onward tickets
Please ensure you have a visa for the Cayman Islands.