All total useage of the 3 phones should not exceed 1000 minutes or you will pay out the butt for "overage."
Usually, calls made to each other (using the 3 phones) is considered "in" calling and don't use any of the 1000 min allotment.
However, I guess that you should check with the provider.
That's how it works with Verizon Wireless...don't know about others.