Although they all three may exist at the same time, what they call "now" is entirely relative.
You're still caught up thinking in a Newtonian sense. In Dr. Greene's book "The Fabric of the Cosmos", he illustrates the point using "now-lists". According to classical laws (and common experience), everyone in the universe should agree upon any given freeze-frame snapshot of the universe. For example, assume I am keeping and updating "now lists". I want to do a "now list" of the position of the sun in the galaxy. I know that it takes roughly 8 minutes for the light from the sun to reach me, and so I can deduce the position of the sun 8 minutes ago. I add that to the "now-list" of 8 minutes ago. The same thing applies for Johnny Alien on Planet Z 10 light years away. He realizes that it takes 10 years for the light from our sun to reach him, and so he adds the position of our sun in the galaxy to the "now-list" from 10 years ago. According to Newton (and Clete), everyone in the universe would agree that the sun was in position X at time Y. Thus, everyone's "freeze frame" of the universe is the exact same snapshot.
However, according to special relativity, two observers in motion will not agree on "now". Imagine that me and Johnny Alien are separated by 10-billion light years (there are two ways to magnify the effects of relativity, increasing the distance and increasing the velocity. This example deals with large distances and small velocities). Although separated by 10 billion light years, assume we are motionless relative to each other. We begin recording "now-lists" of the events on Earth. Because we are motionless relative to each other, our now-events line up perfectly. He records atomic nuclei decaying at the same time I do. Assume that Johnny Alien gets up and runs away from the my position (the earth) at 10 miles/hr. He still keeps-his now lists, except when they are compared, the events on his now-list occured 150 years ago on my now-list. He now hops in a jet and travels 1,000 miles/hr towards earth. The events on his now-list occured 15,000 years in the future, according to my now-list.