Child object collision layer is ignored? Only parent collision layer is listened to?

I have two added collision layers - a and b. There is also the Default layer.

I have the object1 and object2. Object 1 is Default layer. Object2 is collision layer a.

I then have object3 which used layer b. In the project physics collision settings, b only collides with a.

In the above example, the collision does not work. However - if I make object1 have layer a and object2 be default, object3 and object2 will now collide.

I need it to work the first way described above. What is needed for this setup to function?

Thanks in advance!