When rendering to your laptop's integrated screen, both video cards will (or at least can) be active at the same time. Optimus begins to run into problems when multiple outputs are involved, though. My personal experience has been that only the integrated screen's framebuffer is shared by the two GPUs, and external connectors such as HDMI or DisplayPort are connected exclusively to one card or the other. For example, monitors hooked up to the external connectors of my Dell XPS L502 are rendered exclusively by the Nvidia gpu - the Intel HD Graphics adapter is not even aware of those monitors.
If your research indicated that the HDMI port for your laptop was connected to the Intel HD 3000 card, most likely anything rendered to that screen would be rendered exclusively by the Intel card. The difficulties described in this similar question would appear to support this conclusion.
As mentioned in that thread, the NVidia Optimus Test Viewer is a very useful tool in determining which applications are being rendered by which hardware. I suggest experimenting with this tool and both the integrated screen and an external connector to determine definitively which graphics cards can be used on which screens.