They already announced these three DLCs a while ago, it's only now we know the date. The third one (Far Harbor) is supposed to be the new gameplay and missions, etc. This last one is the most minor of the lot, it's just a small $5 installment (settlement improvements, pit fighting, etc).
I got thr season pass for $20 before the game came out so its decent for me, I wouldn't want to pay individually for each.