In the first video of week 4, when depicting the neural networks, the input layer contains x1, x2, and x3. Are these values different training examples entirely or are they different features of one training examples? (For example, if the dataset was about houses, would x1, x2, and x3 be different houses/rows of the dataset or would they be different features of each row(so like number of bedrooms, square footage, etc.))?
Those are different features of a given input sample. But note that Prof Ng has chosen to format the sample matrices (X) with the columns as the samples. That means that the different features of a given sample are all in the same column, not the same row, of the X matrix.