Is there a way to take a contrast medium to the paints to have similar effects to GW contrasts? I know P3 is different in that it actually all pigment based, and just watched a whole Ninjon video about how the GW are contrast paints with 1 pigment…so…is it possible?
Thanks a head of time.
Yes, you can do something like that with P3. 
What I have done in the past is, essentially, this:
Prime a model white.
Wash the model with something to define the crevices. This could be Armor Wash, or it could be something slightly more colorful like a highly thinned-down mix of Coal Black + Battlefield Brown.
Wait for the above to dry.
Hit it from above with what seems like an extremely heavy drybrush of either Menoth White Highlight or Morrow White. It will look very, very weird, but you want that super-high contrast between the edges and shadows. Always brush down, never brush up; you don’t want the shadowy areas to have sharp edges.
Mix roughly 50/50 of the desired P3 color and P3 mixing medium with just the slightest bit of water, and apply the mixed paint to the model, being careful to avoid streaking.
You can skip steps 2-4 if you want and just go straight for the Mixing Medium + Paint (which makes it a lot more like contrast paints in practice, I suppose), but I think the above method gives better results!
If you look on youtube you will find recipes for getting regular paint to behave like contrast paint. They usually involve the use of various mediums and sometimes dishsoap to work as a surfactant. I haven’t tried any myself as I don’t use that style of painting. Besides, when I do feel like taking a shortcut, Army Painter Quickshade Dip works great on top of pretty much any regular paint.