RNN model with 3 hidden layers
The shape should be 3d array --> (samples, timesteps, features)

It needs to add return_sequences=True in first two RNN layers.

model=Sequential()
model.add(SimpleRNN(1000,input_shape=(1,320*15),activation='relu', return_sequences=True))
model.add(SimpleRNN(1000, return_sequences=True))
model.add(SimpleRNN(1000))