Bob Peterson wrote: “I don’t think anyone can give you a simple explanation of what you do not understand.”
But at least, I will give it a try….
Steve Timothy wrote:‘Is there a plug-in or way to do 3-way color correction in photoshop?’
Yes, similarly to the tools in your video-editing software, you can apply CURVES or LEVELS to only HIGHLIGHTS, MIDTONES or SHADOWS. In fact, it is very easy.
Open the photo with the shadows you want to adjust (fig 1)

Add the layer with CURVES. I adjusted the RED channel just for demonstration. You see that the whole picture from black to white is adjusted with red color (fig 2)

Go to the MASK tab in the CURVES layer (fig 3)

At the bottom of this window, choose KLEURBEREIK (or whatever it is called in your language (fig 4).

In the popup window that opens, choose SHADOWS . The example shows that the red color adjustment is just applied to the SHADOWS part of the picture). You can also select MIDTONES or HIGHLIGHTS and just see for yourself what happens.

And ofcourse, you can define yourself what is called SHADOWS, MIDTONES or HIGHLIGHTS. If needed, I can explain how to do that.
Theo