I was in this position recently too, the Data Viz is definitely a step up from the Front End!
As far as Sass goes, to me it is really just adding some efficiency and optimization to your CSS. I use variables and nesting a lot, but it has other features as well. Sass is definitely the easiest of these three to learn and codecademy has a Sass tutorial they released recently.
For D3, this is a really, really good place to start. It walks through the very basics all the way through building some basic charts.
For React, there are tons of tutorials available online but I would recommend starting with Facebook's React docs, and after that this is a good beginning tutorial series, but there are really a lot of React resources so look around to find something you like. Tyler McGinnis has some truly awesome material that is a more advanced.
Hope this helps, good luck!!!