The champ spawn probability is in the hyperlink. It, along with every other info graph about TFT states there is a 0% chance to receive a tier 3 champion at level 3. I've pulled a tier 3 champion countless times as a level 3. I'm not sure if this is intended or not. If it is, i would like to know where to find a proper info graph. But the lack of consistency makes me hesitant to play ranked when it comes out, considering we don't have the proper info to min/max the way we prefer.
imgur link is proof