My husband has a 3-year resident permit in Germany and with his passport he can enter any Schengen country. Can he enter Croatia with his passport?
On Ministry Of Foreign and European Affairs of Republic of Croatia's website:
Pursuant to the Government’s Decision, starting with 22 July 2014, the Republic of Croatia applies the Decision No 565/2014/EU. All third-country nationals who are holders of valid Schengen documents, as well as national visas and residence permits of Bulgaria, Cyprus, and Romania do not require an additional (Croatian) visa for Croatia.
So the answer to your question is yes. Your husband can enter Croatia with his passport along with the German residence permit.