Yes it was cancelled. Too bad.
But given how season 3 ended, the watching experience moving forward would have changed and not for the better. Let me explain - season 3 ended with the start of an invasion by another alien race with whom the first invaders were at war with. Season 4 would have been a special effects shootout which I do not care for. The strength of the series was character development and the humanity of its main characters despite the admittedly far-out plot. Throughout the first 3 seasons the characters, dialogue and plot developments all remained believable - devoid of clichés and silliness that most shows in this genre fall for.
TBH, I am more gutted about the cancellation of "Mind Hunters". That one was building up to a great season 3 revolving around the truly sick BTK serial killer of Wichita, Kansas. Mind Hunters gives me the creeps akin to when I first saw Silence of the Lambs.