use <ALT> <Print> SC (often !) captures several PICS from that same screen.
I think what you are describing is SC's attempt to capture the partial transparency when performing an "Active Window" capture.
In such a case, the result is only a single capture -- but you see it flash on screen 3 times as it tries to figure out partial transparency (for example rounded corners).
You can disable this if you don't care about the partial transparency or tweak the settings by going to the "Window Capturing 1" and "Window Capturing 2" tabs, and disabling the options to try to capture Aero Glass, and the detect active window transparent options.
If I'm misunderstanding what you are describing let me know.