I have a Lenova Thinkpad 410 and a MiniDock Series 3 with two monitors hooked up. All three (including laptop display) screens are detected but only two are able to be activated, how can I get this to three?
I don't believe you can. Either the Intel HD Graphics or Nvidia Quadro NVS 3100M graphics chips you're likely have in the T410 only support two displays.
You'd need to add another video card (like you can do in desktop computers) to get support for 3 or more displays.
First, Check and see what video card your Laptop is using. It is likely one of the nVidia Quadro Mobile flavors.
The Quadro video card (and therefore your laptop) can support two "physical" displays simultaneously. Laptop+external OR External+External, for example through the use of the docking station (which you mention) that also has a two-source output maximum. You could use a video splitter to split one of the outputs between multiple monitors or a USB 2.0 external Video Adapter like the Arkview USB-DH88 PC-to-TV Multi-Display Video Adapter. This USB 2.0 to DVI/VGA or HDMI Adapter enables you to connect up to 6 monitors, LCDs, TVs or projectors to your PC or notebook.
You may however have up to 16 "virtual" displays if you have and can use the NVIDIA® nView™ Multi-Display Technology software.