Chemical composition of all 3 is here - http://www.zknives.com/knives/steels/steelgraph.php?nm=420hc%2C13C26%2CCPM%20S30V
Although, if you are planning on knife use around salt water or area with very high humidity then 420HC might be a better choice, it resists rusting a little better than the other two.
Sandvik 13C26 is also a very fine steel, used in razor blades as well, it can be sharpened to very fine edge,when heat treated properly that is.
CPM S30V is Crucible Particle metallurgy steel. Hi-tech alloy in other words. Developed by Crucible in collaboration with several knife makers including Sal Glesser, Phil Wilson, Jerry Hossom, Chris Reeve, Rob Simonich and few others. It didn't really come out all it was hoped for, but as of today, it is the premium stainless steel for many factory knives.
More on those steels in Knife steel FAQ - http://www.zknives.com/knives/articles/knifesteelfaq.shtml
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