After training my network, I have got similar cost-errors for training and development data-sets, and twice as large cost-error over the test set.
I used the cost-error over the development data-set to tune the hyperparameters.
This looks like the overfitting of the training and the development data-sets.
How can I improve my network, to get the cost-error over the test data-set down to the level of the cost-error over the train/dev data-sets?