So I bought Witcher 1 on Origin, and Witcher 2 and 3 on GoG. Before I install and play, is there any special way I should install them, or do anything profound to make sure I can import saves properly from one game to the next (the whole saves carrying over from game to game feature)?

Don't take this as gospel, but I'm almost certain that The Witcher always writes saves to your Documents directory, regardless of where you bought it from, so you should be able to import into the next game without issue. Also, you should be able to redeem your Origin key on GoG to get a copy of it there if you want to go that route.

Keeping everything in one place would be best! Thanks, I'll and do that.