How do I connect 4 displays with a late 2015 iMac (two mini-display ports, usb 3.0, no usb-c)

I'm trying to run ProPresenter 7 with three external screens showing separate content on each (plus the internal screen).

Before I buy anything I wanted to see if anyone else has any ideas. My plans were to do one of the following:

-USB 3 to HDMI adapter

-USB 3 to USB-C hub and use one of my USB-C to HDMI converters

From what I've read, having a mini display port to 2xHDMI would just make duplicates of the screen so that wouldn't help for what I'm trying to do.

Any thoughts? Thanks in advance!