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It's in the "Legacy" group, and it's an old effect. The Lumetri panel does many things easier & quicker, though occasionally one can do something in 3-Way you can't easily do otherwise. I never use it, haven't in a few years.
Just enter "3-Way Color ... " in the search box at the top of the Effects panel and it will pop into view in the list.
Neil
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Neil is 95% correct - I'm sure you've already found it, but technically the folder it resides in is called "Obsolete" not "Legacy." 
Legacy does sound nicer though! Also, the Effect is called Three-Way Color Corrector, not 3-Way Color Corrector... I'm not normally that picky but I tried searching for it using the number 3 and it doesn't come up! Adobe is particular
Thanks for catching that, Jason ... on my laptop, wrote before checking. Bad Neil! Bad Neil! 
I'm so used to using Lumetri/Ripple (or Elements) I'm getting rusty on the old-style effects. And older, of course!
For the original poster (OP), most YouTube/Vimeo free tutorials featuring the Three-Way or Fast color corrector effects are a few years old ... and are in the Obsolete "section" for a reason. As I noted above, there are a couple things easier to do in them, but in general ... the Lumetri panel is a better tool. Look around the web for Lumetri tutorials.
Neil
Ann's got a good point, but if you want to talk about steep learning curves, AE's graphic is right there on the first page ...
Though applying a "simple" filter is pretty easy to do.
The Color Workspace/Lumetri panel has numerous tabs ... designed to be worked pretty much in order top to bottom.
The "Basic" tab is built to look very Lightroom-ish, Temp/Tint, with a color-picker for "auto white balance" (though in 2017 the name by the picker [eye-dropper icon] is wrong ... ignore it, just try the picker on something that should be grey or white).
The blacks/whites sliders do exactly what you expect, the Exposure slider is a mash-up of gamma/Lift/contrast/brightness controls that lifts the middle tones a titch more than others, but will eventually move everything.
Shadows/Highlights controls mostly move what they're called, but do a wee bit on the entire scale so you typically use them against each other.
"Creative" tab ... a funsies place.
"Curves" tab, with four different curves controls on one user interface and a separate Hue/Saturation circle control. On the Curves device, the highlighted color dot at the top is the color you'll be adjusting. If it's the white one, it's everything, a colored dot, just that color.
"Color Wheels" tab shows the basic colorist's interface of three "balls" with a vertical slider. The balls are the chroma (color) change tools, the sliders the luma (brightness) change tools. Left to right, shadows-midtones-highlights. Click in a ball, and slide the mouse ... your colors go the direction of the colors on the typical color wheel. You can watch this change best in the Lumetri Scopes panels, which you should ​always​ have open while doing any color work. I use the RGB Parade, Waveform (either RGB or Luma) and Vectorscope (YUV) always.
This is an easy tab to use, very simple and effective.
Then the "HSL Secondary" tab gives a spot to do ​targeted​ changes. You use the controls on the top to select a "key" (by selecting parts/all of each of the Hue, Lightness, and Saturation controls) and modify the pixels picked by that key using the tools at the bottom ... color wheels, temp/tint, contrast, sharpness, saturation.
And of course the "​Vignette​" tab ... pretty self-explanatory.
Neil
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