Flanery says they’ll start production and filming in November 2015 to April 2016. Add a bunch of months of postproduction and distribution to it, and we could get it in late 2016.
However, as with all Boondock III news lately, take that information with a grain of salt. Remember that we are talking about Troy Duffy being in charge here, and he’s been saying shit since 2011. For all we know he could suddenly change his mind again and think it would be best to publish a novel or encyclopedia first or something.