Brandon has begun writing on Stormlight 3 in earnest, and today Brandon revealed that the flashback character for this Stormlight book will likely be Dalinar. This may come as a surprise to some, as the previous working title for this book, Stones Unhallowed, as well as Brandon's initial plan suggested Szeth was to be the flashback character. However, more recently, Brandon said that it could be either of the two, or Eshonai, depending on which information needed to be revealed first.
While Brandon hasn't officially decided, he won't know for sure until he finishes writing the sequence, he is definitely "leaning that way".
The other flashback characters in the first five Stormlight books will be Eshonai and Szeth. Since the original plan was a Szeth, Eshonai, and Dalinar order, it seems likely that we will have Eshonai for book four, and Szeth for book five. That will be it for the first half of the ten book Stormlight sequence. There's a time jump between books five and six, and Brandon will take a break from Stormlight books to do some other projects, like the Elantris sequels and the modern-day Mistborn trilogy.
Update 1 to include info that it is not 100% certain just yet.