is this really an issue?...you have three computers (2 of which need a monitor) and you have two monitors. one monitor for the mac mini, one monitor for the pc, and and the mbp has a built in. i dont see the reason to leave a computer in the dark so you could screen span.
if you NEED to screen span..im almost certain the dell displays you have has both a vga and dvi slot each (controlled by a button on the front of the display). if this is the case, plug you mac mini (via dvi) to one of the displays and connect your pc (via vga) to the same monitor (so now you should have one monitor with both your mac mini and your pc plugged into it, each occupying the dvi and vga slots respectively). then plug your macbook to the second monitor (via dvi) and plug your pc computer to the second monitor as well (via vga) just like you did with the first display. so now when the monitors are in dvi mode, you could use your mini with a solo screen and your mbp with screen spanning, and if you want to fire up your pc...just press the buttons to switch the monitors from dvi to vga and voila...two screens dedicated to your pc.