I figured out my problem and the easiest way to do what I want is to buy an USB Display adapter with DVI to to VGA plug.
I bought one for about 50$ and now I have only one laptop ACER 6920G with video card GF 9500S wich support 2 displays and the third connected to the adapter.
I bought something like that:
And it looks like that now:
I hope someone finds it helpful.