How to handle 3 dimensional layers in TensorRT?

According to this thread (see developer “NVES” response) you have to “[…] remember to register your input blob with NCHW format”. It sounds like TensorRT is expecting a 4 dimensional input. However, the official tensorrt sample code for uff seems to use a 3-dimensional input. ( line 170 nvinfer1::Dims3(1, 28, 28)).

Does TensorRT automatically append the fourth dimension, like converting 1,28,28 to 1,1,28,28 or how does it work?