A Certain Scientific Railgun T is unlikely to help you understand A Certain Magical Index III. I've watched the anime for Index, and around half the episodes for Railgun T. Additionally, I've read the manga for A Certain Scientific Railgun, as well as reviewed summaries for the remaining episodes. I strongly suspect that the remaining episodes will closely follow the manga as the first half of the season did. Thus, I can pretty much infer Railgun T isn't going to help. These arcs pretty much tells their own story, and don't have much to do with the story told in Index.
Trying not to spoil anything, but given the contents of episode 68 of Railgun, I'd actually guess that it'll be helpful to watch A Certain Magical Index III first. And watching A Certain Scientific Accelerator before A Certain Scientific Railgun T will be helpful because some characters from there show up in the second half of Railgun T.
If you need more info, this question has some good answers about watch orders: Watching order for "Toaru Majutsu no Index" and related anime