Immigration Services
Before you even think about getting Serbian citizenship you need to already possess permanent residence permit, and for that one, you needed to own a temporary residence permit at the beginning.
More precisely, you can apply for Serbian citizenship if you already have the permanent residence permit and you have registered living address in Serbia for five years after you got your permanent residence status.
What is said covers most cases, including those when foreigner lives in Serbia due to the ownership of the company or employment in Serbian company, while for marriage law stipulates a bit different case.
Marriage with Serbian citizen (or holder of Serbian permanent residence permit) gives the right to a person to apply for the Serbian citizenship immediately after being granted with permanent residency without the need for 3 years wait.