fix padding bug in pooling3d

Author: oeway

keras/backend/theano_backend.py

@@ -802,7 +802,7 @@ def pool3d(x, pool_size, strides=(1, 1, 1), border_mode='valid',
         raise Exception('border_mode="same" not supported with Theano.')
     elif border_mode == 'valid':
         ignore_border = True
-        padding = (0, 0)
+        padding = (0, 0, 0)
     else:
         raise Exception('Invalid border mode: ' + str(border_mode))