3. The man arrived at the station but the train had already left/already left.

3. The man arrived at the station but the train had already left/already left.

Поскольку прибытие на станцию произошло после отправления поезда, для указания предшествования нужно использовать Past Perfect (had + V3).

Ответ: The man arrived at the station but the train had already left.