I know this is an old post, but I researched this a while back and couldn't find a a mod or any other convenient solution up until now, so I'm here to present it to whomever might look it up in the future.
If you want Geralt to auto walk/run/sail because you work with your hands, has an issue with your nerves or want to avoid it, there's a glitch you can exploit. It's a bit immersion breaking but it's worthwhile to be able to enjoy the game while avoiding pain and health issues. Imho. Here it is:
1) Download the latest Reshade for TW3 whether you intend to use it or not and install it. (it's not on nexus, btw)
2) Open it when ingame and, in the settings tab, assign a key that's easy to press and is not used otherwise.
3) While ingame, whether walking, horseriding, or sailing, (a) hold the forward key, then (b) press the key you assigned to open Reshade settings, (c) let go of the forward key, (d) press the key you assigned for Reshade again to close it.
Voila! Now Geralt auto walks/runs/sails. I stumbled upon it after giving up on finding a mod or macro that would work and my left hand has been thanking me ever since. Happy, healthy, pain-free playing to you all!
PS: a bug might happen if you do the process described in *3 while the camera is being forced to turn by the mouse where he keeps turning in that direction. Careful to be pressing only "forward" (W) to do it right. If you get it wrong you can save, exit the game to desktop and then reload and it'll be fine.