How to enable 3 screens on HP EliteBook 8570p with intel cpu and HD4000 integrateg graphics card?

I'm trying to enable 3 displays in EliteBook 8570p. I see that 3 monitors are detected, builtin and two external connected into dock station with DVI. I tried reconnect directly into laptop as DVI / VGA without result. Always one of three displays is grayed. Computer generate video signal only for two monitors, it is no matter which one of them.

I'm using Windows7 pro. Computer has APU, Intel i5-3360M 2.8GHz with Intel HD Graphics 4000 .

Making extremely low resolution (800x600) gives no results.

UPDATE:

First monitor is laptop integrated display. Two external monitors are connected via DVI to dock station. I tried to connect one of them by VGA analog classic cable to dock station, or reconnect it to port in VGA socket in laptop body, but it behaves the same. The same is when one of monitor is connected into DisplayPort with HDMI-DP adapter, because no one monitor I have got, has no DP socket.

I thought serviced pixels are main problem, but all is the same when I go down resolution on all monitors into 800x600 pixels. Dock station doesn't have any graphics, it works as adapter for monitors.

Some of people told me, it is impossible using 3rd monitor with integrated graphics, but I saw some info on internet that it is possible. I didn't test it with HDMI, because I haven't got any equipment for this, but dock station has got two HDMI sockets.