Hmm, probably Cleric, Blacksmith, and either Potter or Shepherd as the third, if playing Ascendency. Blacksmith is the most useful of the base game crafters atm in my opinion, Potter/Shepherd are fairly useful but fairly annoying to unlock, lol
Also because I play with extensive mods, so Blacksmith and Potter tend to have some good content to them; Blacksmith is also still useful as a worker.
Blacksmith also makes the Carpenter’s Saw, which thus loops you back around to the other crafting classes fairly quickly as long as you get the Blacksmith going ASAP.
If you add in mod classes, then Miner replaces the third one, hands down. @Moai did an amazing job with it.
Even if you add in my own WIP modding, which is aiming to add a suite of useful little machines to the Engineer, still not that hard to up the Blacksmith into the Engineer.