Exact equivalent of 15Mo3 is not possible to obtain. However, the nearest ASME equivalent for plates is SA204 (Gr.B is more commonly used).
I would appreciate if you could tell me your application as well. It is because C-1/2 Mo(which is how you would express this material) is hardly being for new constructions. 1 1/4 Cr-1/2 Mo is the preferred choice for substituting an erstwhile C-1/2Mo selection.
Hope you find this information useful.