Optiplex 7070 SFF - Connecting Three Monitors

Are you certain on the model 7070 SFF?  As you can see from picture below, it has 2 displayports (#6), a serial port (#3), and an optional port (#5), which can be DisplayPort/HDMI 2.0b/VGA/or USB Type-C Alt-Mode

Which number were you referred to as HDMI and VGA?

This is it.  The HDMI Is plugged in there. I now have another monitor plugged into the Display Port above the HDMI.  Mine does not have two Display Ports. I got the model number by entering the serial into this website so I’m sure it is correct.  

Okay, you got the correct model of 7070 SFF.  Also, I think I found the issues.

1.  You are confusing the serial port (#3) to a VGA port.  Hence, you have difficult time to find a suitable cable.

2.  The video cable in your picture actually is a displayport to hdmi cable.  You can see displayport logo, a D with a P inside.  They were imprinted on the side of the port and on the cable connector.

The solution for a third monitor is using optional video adapter (#5).  For example, an optional HDMI I/O daughter board.  It does require to open the side cover to plug the connect video adapter to mother board and secure the video connector to i/o shield.

Sounded like a headache. I just bought a 34” monitor that can function as two monitors. I don’t know why Dell made a computer with a thousand USB ports and so little else though. 

That ultrawide monitor should simplify your setup and resolving the connection issue.  Every other computers in the market are having similar built. 

Contrary to your preference, many users are looking for ways to expand the connection ports for their setup.