We've heard rumors about this for a while, but now it's finally official: the release date for How to Train Your Dragon 3 has been pushed back one year, from June 17, 2016 to June 9, 2017.
There is some speculation that Finding Dory's June 17 release may have been the reason. It's possible that this influenced DreamWorks' decision. However, the date change appears to mostly be because Dragons 3 is a bit behind schedule. As of Dragons 2, neither the story nor voice work was complete, as was planned.
Ending a trilogy is hard, so this makes sense. It just means that when the film comes out three years from now, we can expect something pretty amazing.
Thanks Molly and janinka11!