To summarize -
You can't use the laptops as a simple direct-connect monitor, but you can use them to control the Mac using VNC software.
The Mac has this software built-in. You'll need to borrow a monitor and keyboard to set up the Mac for this. And the Mac and whichever PC will have to be on a local network. (Ethernet is better/faster)
On the Mac, go to System Prefs > Sharing, and turn on Remote Management (I also have Remote Login on).
On the PC, use a VNC "client," and use setting for True Color (24 bit).
(I use this every day for my Mac mini in the basement.)