Question

How do particles' varying reference frames average to a single macroscopic reference frame?

Answer 1

time must behave differently in frame A and frame B i.e. there is some absolute sense in which it's slower in the moving frame and faster in the stationary frame.

Suppose you're sitting in frame A watching me in frame B. As far as you are concerned you're stationary so time moves at the normal rate. However you see me moving, so you see time moving slowly for me.

But now look at it from my point of view. As far as I'm concerned I'm the stationary one, and you're the one who's moving. That means my time moves at the normal rate and I see your time running slowly.

This means the situation is symmetrical. Each of us sees our time move as normal and the other person's time slow down. Actually it has to be this way because if the situation wasn't symmetrical there would be a way to assign absolute motion i.e. one frame would be different to the other.

The third observer (I assume you mean he sees us both moving with equal but opposite velocities) see both your and my time slow down. Likewise we both see his time slow down.

In general you need to very careful with intuitive arguments about relativity, because it's unintuitive

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