I went over the data science curriculum on FCC, and I was wondering if anyone had recommendations on resources to understand D3 better

I'd start by clarifying that FCC resources are on one data visualization tool, not on data science overall.
Data Camp and edx are good starts with free introductory-level resources on data science. If you want to learn D3 to be come a D3 master then I'd start with FCC's JavaScript curriculum, as D3 is first and foremost a javascript library