The missing of being able to copy a Mathcad plot in vector format to the clipboard was criticized a lot here in the forum. I am not sure if this is to be expected in Prime3, though - I fear no.
The only way at the time is to take a screen capture and use the bitmap (horrible and unacceptable) or copy the data to Excel, Origin ar whatever and do the plot there.
PTC has to come up with better export posibilities and better ways to format plots hope fully soon!