Thanks for your reply. Yes, that seems to be the issue. Inspecting the values confirms your thought that the dimensions from openCV do not match.
I see two possible solutions:

Create three 2D tensors and merging them (instead of merging in OpenCV and creating a tensor afterwards). How can I merge the tensors?
Reshaping/Sorting the existing tensor which was created using torch::from_blob. How can I reshape the tensor?

I guess that I can find the answer to both questions here: https://pytorch.org/cppdocs/api/namespace_at.html#namespace-at 11
However, the documentation is rather limited. I could therefore need a hand pointing me to the correct function. If possible, maybe a little example