3-pixel horizontal scrolling on the Atari is possible! Normally the playfield limits scrolling to increments of 4 pixels, but because 1 cpu cycle is the same amount of time as it takes for the TIA to output 3 pixels to a scanline, the colour can be modulated in 3-pixel increments by writing to BGCOL (or FGCOL) every single instruction. This sequence of instructions can be pre-computed and executed from RAM, as in this demo.
More info: https://forums.atariage.com/topic/341661-3-pixel-horizontal-scrolling-homebrew/
3-Pixel Horizontal Scrolling (Homebrew)
The other day I talked to someone who had an Atari, and I got interested in doing homebrew, which I've never done before. Attached are gifs of my first demo (purple), and a 3-pixel scrolling example which could be interesting to some. Normally, the playfield only allows scrolling in 4-pixel incre...