Here's a Sonic 3 theme poster, with not-quite-on-model Knuckles.
As you can see he has sort of jelly-bean feet, short quills, a wiggly mouth and an enormous nose. Also, his chest stripe is mis-colored as yellow. The "3" from the title of the 'you need Sonic 3' Uncle Sam style poster above appears here as a giant background over a rather barren terrain.
Remember, at this point Knuckles was still pretty much an antagonist for Sonic, so it's interesting they have him flailing around in the background.