From the blog (http://blog.sourcetreeapp.com/2013/05/07/sourcetree-for-mac-1-6/#comment-888864641):

"I can't get Araxis Merge to work with an actual 3-way merge. Launching an external diff works properly, but the "Resolve Conflicts->Launch External Merge Tool..." just quickly opens the external merge window and then closes it. I also see the <filename>.orig files building up in the folder.</filename>"

Author to provide more details.