Nvidia or AMD, set the in-game AA to off, you want to set it from your graphics card control panel (download Nvidia Inspector for Nvidia GPUs).
In NVidia Inspector use AA 4xS, 8xS, 12xS, 16xS, or 32xS. (32xS is probably too much, especially on triple screens, probably same with 16xS, but it doesn't hurt to try) The LOD bias clamp thing hasent worked for a couple years now as Nvidia has disabled it (unless, I think, you have a real old card like a GTX 280 and/or real old drivers) that's where these "xS" settings come into play as they have their own different LOD bias than other AA settings. I'm not sure if it matters, but I leave "Texture Filtering - Driver Controlled LOD Bias" to ON, and "Texture Filtering - Negative LOD Bias" to ALLOW, just in case. You will get tons of jaggies and shimmering no matter what settings you use in rFactor 1 based sims (and pre-rF1, eg. GT Legends) unless you use the "xS" AA settings in Nvidia Inspector, or possibly the method below.
The combination of Multisampling AA + Sparse Grid Supersampling (make sure they are both set to identical amounts, eg. 2x MS + 2x SGSS, or 4x MS + 4x SGSS, or it won't work properly, they need to be the same amounts) is another option but it doesn't have the different LOD bias that the "xS" settings have, but you may still get good results, especially with 4x MS + 4X SGSS.
For AMD you just use the standard "Supersampling" option in CCC (Catalyst Control Centre, your AMD graphics card control panel) and then go with either 2x or 4x. 2x will still have some jaggies but overall very good because AMDs supersampling will apply to the entire image, no "ifs", "ands", or "buts". 4x will look beautiful, you won't need anymore than 4x SS. 8X will look just a teeny, tiny bit better than 4x but it is much more demanding than 4x and not worth it; 4x is very close to the IQ of 8x. Just use standard 2x, 4x, or 8x, don't bother with the "EQ" settings (eg. 2xEQ) and don't bother with edge-detect for the "Filter" option.
P.S. Make sure you have the AA in AMD CCC or Nvidia Inspector set to Override application settings or none of these settings will be actually used and you'll be left with no AA (since the GSC config.exe should be set to no AA).