Conditional Type II and Type III

The first sentence is an example of the Third Conditional. It means that the speaker has not eaten breakfast and (s)he is hungry now as a result.

The second sentence is ungrammatical, because the modal verb would does not normally occur in the if-clause in a conditional sentence. The grammatical form is ‘If my brother had been there, he would have known what to do’, and it, too, is an example of the Third Conditional.

As written, the third sentence might be possible, but it might better be expressed as ‘If it hadn’t been X, it would have been something else’, and it, too, would then be the Third Conditional. Alternatively, it could occur, with a different meaning, as ‘If it wasn’t X, it was something else’.

The categorisation of conditional sentences into First, Second and Third is a useful starting point, but, as Peter has said, it is a simplification. Other combinations of tenses are found in conditional sentences which can express, for example, promises and offers, or threats.