Absolute position, 2D pos of 3D layer

From within the same layer, use this:

toWorld(anchorPoint);

From another layer it would be like this:

L = thisComp.layer("child layer");
L.toWorld(L.anchorPoint);

will also work as 2d null position of a 3d layer.

to get this working in separate dimensions:

L = thisComp.layer("child layer");
x =L.toWorld(L.anchorPoint)[0];

with [0] being x pos, [1] being y, [2] being z