Why not three PSU?
Since you already have 860 and 750, I would add another 750 or 860 and get a better night sleep knowing that you have plenty of headroom and better PSU efficiency.
You can sync them all using something like this:
https://www.amazon.com/gp/product/B072K8F6GZ-SCSI
Why do people use stuff like this? It's very simple to use them all together, you simply need to short the green pin to any black pin in the ATX connector and the PSU will be on and providing power. Here is a very simple guide:
http://www.instructables.com/id/How-to-power-up-an-ATX-Power-Supply-without-a-PC/It even shows using a switch, but I just prefer using the power switch on the back of the PSU. I have been doing this since 2014 with no issues ever.
As far as I understand, stuff like this (a relay or a transistor based switch) will provide an additional protection in some failure cases. For example, if it fails, it will always fail in an off state while a simple green wire connection will leave the slave PSU(s) at the mercy of the primary PSU. Also, a relay based sync switch may provide complete galvanic isolation between PSUs if its desired.
And finally, yes, in most cases connecting green wires between three PSUs will work, but it requires some wiring or soldering skills and a bit of luck.
