Considering that Ubisoft has no current plans to release a new Assassin's Creed game this year (it's hard to count Assassin's Creed III – Remastered as a new release since it's a slightly modernized version of the 2012 title), there's a big chance that the Watch_Dogs series will return in 2019. As you probably remember, the last time that long-running series took a break was in 2016, right after the release of Assassin's Creed Syndicate, with Watch_Dogs 2 taking AC's place in Ubisoft's release schedule.
Back in April, Ubisoft’s gaming assistant AI brought players the ultimate proof that Watch_Dogs 3 exists, saying "from the last early build I tried it’s very solid. The Dev team works wonders! Can’t wait for you to try it!" That wasn't the only time Sam has revealed new information of Watch_Dogs 3, as the AI said about two months ago that "They" will come. Or "He". There's a debate about that."
As reported by YouTubers UbiCentral and Skullzi TV, Ubisoft recently filed a new trademark for Watch_Dogs, hinting at imminent announcements related to the hacking series. The fact that Ubisoft's new games such as Far Cry: New Dawn, Trials: Rising, ANNO 1800, and Tom Clancy's The Division 2 will be released in the next couple of weeks suggests that Ubisoft will reveal WD3 rather sooner than later.