Dragons 3 is coming in 2018. it would've came early, but due to their financial problems they are only putting out 1 or 2 movies per years instead of 3 or 4 like they did before
DreamWorks tends to make its own animated films into series: Trolls, Puss in Boots, Spirit, the Croods, etc. This may work out better like a series, if they do it like Rugrats.