@whatisthis is Correct, in each elemental summons, you have the same odds of drawing heroes of the same tier. The highest odds are obviously the 3* of that element, followed by the 4*, then the 5*.
The featured heroes are always the 5s of those respective elements as a form of enticement for you to make the draw, so for yellow, you might start seeing banners featuring Leonidas, Joon, Justice, Vivica on rotation, but the odds of drawing them are the same and you can see the odds in the little exclamation mark on the top left corner before you draw. Even tough 3s and 4s are in the mix, it won’t be a very enticing ad if it shows featuring Gan Ju now would it?
Gan Ju: Aww, that makes me a saaaaaaad Panda.
Hu Tao: It’s ok bro.