[Gabriel Grenier]

I’ll put the working solution here for future reference.

The following expressions assume that the target 3D layer is the next one, right under the 2D layer (index+1), but that could be changed of course.

Expression for position:
thisComp.layer(index+1).toComp(thisComp.layer(index+1).transform.anchorPoint)

Expression for Scale:

L = thisComp.layer(index+1);

C = thisComp.activeCamera;

CP = C.toWorld([0,0,0]);

LP = L.toWorld(thisComp.layer(index+1).anchorPoint);

dis = length(CP,LP);

CZ = C.cameraOption.zoom;

LS = L.transform.scale[0];

S = LS*CZ/dis;

[S,S];