Merging 3 brackets?

…how do you determine the threshold accurately?

HDRMerge: latest from master (09/08/2016)
RawTherapee: 4.2.1002 (latest RPM for Fedora 23)
raws: https://drive.google.com/open?id=0BwaAtx_CNzb6dEpMQVYtY2dPQ28

There is nothing special about the shots, I just want to play around with merging and raw edition.