Citizenship criteria

Hello, Say someone landed in Australia on 1st Jan 2016 on a Permanent Residency 189 visa. When can the person apply for citizenship? Am I right citizenship can be applied on 1st Jan 2020 provided the person completes at least 3 years of stay in Australia amongst those 4 years and provided last 9 months were in Australia? Is there a way to do this sooner? What if you have a job offer or a written contract by an Aussie employer allowing you to work from overseas? Also, after application

Thanks for the reply! If the applicant does not have a permanent home in Australia after application for about 3 years, do you think this could be an issue? Like you at least would come home for 3 weeks every 9 weeks. But what I'm asking is that the applicant will be here till say Jan 2020 and then not have a home in Australia for 3 years. He/she is willing to be in Australia to attend the test and ceremony and may come for a brief visit other than that, but that is about it. Is this alright?