There's an interesting story here. It's true that the orbits of planets in the Solar System aren't quite stable - they're just pretty stable, but that's good enough for us. So I suppose that it is possible that Mars might collide with Earth in about 3 billion years.
But let's get a bit of perspective. According to evolutionary biologists, the most complex organisms alive 3 billion years ago were single-celled organisms too small to see with the naked eye. What people would be like in 3 billion years time is an interesting question, but it's pretty likely we wouldn't be much like we are today. The total of recorded human history is about 10,000 years or so, before which time (if we existed at all), we were living in caves and hitting things with sharp rocks. 1 billion years is 100,000 times as long as that. So the whole of human history, back to back and left to run for 300,000 times. That's how long 3 billion years is.
But that's not the point either. Because even if Jesus hasn't come back by then, our understanding of how stars work suggests that the Sun will by that stage be a fair bit warmer. So much warmer in fact, that many predict that by then the Earth will be too hot to have important things like liquid water on the surface and we'll all be dead anyway, unless we've invented some clever spaceships or something.
This page, for instance, suggests "the end of large surface life on Earth" in about 1.1 billion years due to how the Sun is expected to change with time.
So given that, I don't think that we need worry too much about being crashed into by Mars another 1.9 billion years after the Earth has become uninhabitable.