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With the density, I would however think you should also take proximity into the equation. Have the 3 words as a phrase in the relevant spots.
I have a site that ranks #1 for our best 2 word phrase if it is singular. Add the "s" and all the way to #70 something. It's tough to get them all in there, just keep an eye on your logs and see what is working and always experiment.
By the way, I'm not an authority on this, It's just my 2