Rotation 3D-2D mode

hello,

I have 2 modes in my threejs app - 3D and 2D.

The 2D is created with different js library. In this mode I want to draw
a shape like in the 3d and rotate it with the same angle (only x,y).
How to calculate right angle based on euler rotation? (from threejs 3d object)

Thanks for help:)