This is my top tip for doing mass HDR Merge processing in Lightroom.
Work backwards.
HDR Merge using Lightroom
Seriously – start with the last set of three images and work back to the beginning.
This gets rid of that annoying thing where Lightroom nudges you along one after it has created a new Dng file, and when you get to the end you realise that you have missed a few.
Seriously this is a great little time saving tip which I am using as I write this!