They're just using words, but it doesn't actually make sense. They are all one, but they are all separate, lol. That's just obviously a made up concept, like you would see if some fiction book. How can 'god' be confined within three separate entities? Why did Jesus talk to God, if they are both...
Now I know a lot of atheists have more reasoning than that, which is great, but in every "why don't you believe question", the typical answer is, "there isn't any evidence". I think that's just insufficient. The huge prevalence of believers should require you to actually have reasons to believe...