Mask doesnt move when 3D composition created

We need screenshots with the modified properties of your layers revealed so we can help you.Select the layer or layers giving you problems and press the U key twice then PrintScreen and paste to the forum. If you are on a Mac then it is Shift + Ctrl/Cmnd + 3 then drag from the desktop.

I cannot tell for sure because there is nearly an infinite number of ways you could create, mask and animate 3D layers but I think you have the false assumption that you need to make the nested composition (pre-comp) a 3D layer. Normally you should not. If you create a mask on a nested comp that has 3D layers and collapse transformations turned on that mask will not stick to the 3D layes in the 3D comp because they are rendered before the mask is applied. If you make the nested comp a 3D layer then you will get othr behaviors that are hard to predict.

I can not point you in the right direction because I have no idea what you are trying to do with the mask or how your comps are set up. If you want a mask to stick to a 3D layer then that mask must be on that 3D layer. I hope this helps a bit. If you can't figure it out we need screenshots and a detailed explanation.