I have a T420 with the Optimus graphics card, a Mini Dock Plus Series 3 170W, and two DVI monitors. I would like to use both monitors as well as the screen on my laptop, all at once.
The guide page for the dock says that, for three monitors, you need: "any combination of the following..." and lists the ports. But, this description is ambiguous as to whether you can use two DVI ports at once, or whether you can only use one of each of the ports.
When I plug in both monitors and open my laptop, one of the monitors shuts off.
Refer to this article on our support site. Check that you have Optimus enabled on your system in BIOS.
You will need to run the third monitor from the VGA port on the dock, or from the display port from the laptop.