I'm getting a new Lenovo T520 with Intel integrated graphics.
Is it possible to use three screens with Intel HD 3000 (the laptop internal screen + two external screens)?
No, according to Intel's FAQ on the HD 3000 integrated graphics:
However it seems you can use a splitter to "clone" a third monitor.
Keep in mind cloning only mirrors exactly what is on one of the two monitors it will not be an independent third monitor.
As allready answered you just have 2 concurent outputs. But you can split it (it's not mirroring).
So you take a big resolution (e.g. 3200x1200) and split it into two (e.g. 2x 1600x1200). You need to have these resolutions (there are some possibilities). I done it years ago with tripple split (1 output to 3 monitors) for creating a cave with one graphics card.