Training The Image classifer cats dogs with one more class

I am using the following code:

Where he does a simple implementation of cats vs dogs. I have used my own imageset which consists of just 240 training images and 50 test samples and I have tried this same model. The only thing I have changed in the model is this

img_width, img_height = 432, 288

nb_train_samples = 480
nb_validation_samples = 100

For two classes good and bad in 5 epochs its reaching 100% validation accuracy. When i add a third class into this for Model/Train/Mixed and Model/Validation/Mixed. Then its giving me a a steady accuracy of 33% only and no improvement from there. Where am i going wrong?

For the code: