'A' is used before singular nouns used in a general sense. 'The' is used before specific nouns. Here, in the first and third sentence, 'radio' is used in a general sense. Therefore, 'a' will be used before them. In the second sentence, we are using the word 'radio' to represent the whole class to which it belongs. In such cases, we use the definite article 'the'. Therefore, the correct option is: a ; the ; a.