How to attch three objects/layers/comps with one nulll

Are you trying to use just two graphic layers, have them move up together, and limit the total number of graphic layers to three, as in your second screenshot where the layers are moving?

If so, by far, the easiest way to make that work would be to use four layers, two copies of each graphic, and create a comp where graphic one is at the bottom and stationary to start with. Then, it moves up to reveal layer 2. The copy of 1 moves off-screen, layer two moves to the top, and layer 1 moves up to the bottom of the frame. 

You then set an in and out-point for the Work Area where Layer 1 has stopped moving to the bottom position with Layer 2 above. The first and last frames of the work area must be identical.

Next, set the in and out points of each layer to match the work area, grab all of the layers, and Pre-compose, moving all attributes to the new comp and trimming the new comp to the layer length. 

The next step is to apply Time Remapping to the pre-comp, which is now a nested comp in your main comp. Now move to the last time remapping keyframe using the K key, then go back one frame using the Cgtrl/Cmnd + right arrow key and add a new time remapping keyframe. Then move to the original last keyframe again, where you should not see the animated layers, and delete the original last keyframe. 

The last step is to add a simple loopOut() expression to the time-remapping keyframes and then extend the layer to the length needed to create your loop.

If that is not what you want to do, let us know. It's easy to use an expression to automatically animate layers like live texting conversations, and you don't even need any keyframes.

I recommend that you review how to  precompose and the lookout "cycle" expression.