3 monitors question - 2 iMacs + Pc monitor

What do I do to have 3 monitors display all at the same time?

I can get my 27" iMac (High Sierra) to my 21" iMaC -10.11.6 El Capitan to display ok actually with no problem.

But when I use my pc monitor (which is plugged in to the back of my 21" IMac ), It does't show up in preferences

of my Main 27" iMac. Nor does it show up in 21" iMac. I know this because in Prefrences - "Displays" in the "Arrangement" section I don't see 3 monitors ! I should have 3 monitors show up that I can drag to the right or left etc. And then be able to change that strip of white to the various windows etc.

What am I doing wrong?

Again my two iMacs show up and work in tandem, but the pc monitor wants nothing to do the the 2 iMacs.

The PC monitor chord to hook in the back of my iMac is into the 21" iMac to PC VGA! ! But it doesn't work in the 27"iMac to VGA-PC either.

Sorry If I am repeating myself !

Bill