How to compute accuracy for CNN when outputs are one-hot encoded
Predicted class is the one with highest probability in output vector (class B in your case) &amp; accuracy is correct predictions %, unless I'm missing your point.

The problem that you have mentioned is representative of multi-class classification which is solved using Softmax output layer in neutral net. NOTE that Softmax uses Cross Entropy as loss function.

I feel that calculating Cross Entropy on output may not make sense.