Like @KariyamaKiyoschi said, witch doctor spawns in the summoning place.
Then Multigun and Jim spawns in sinners refuge. Very easy alchemists!
Yet many camps spawn alchemists, Sepermeru has multiple spawns, Volcano in the noxious garden, mount of the dead, pagoda, Buccaneers bay, new Asgard, etc…
If you need a fast tier 3 go to Buccaneers bay and buy some of them, they cost only 50 gold and you have a good potential to gain a named one as well, good luck.