Great question - Easy answer - the Buck is by far the best value for the money of these 3. You get S30V steel (better than the steel in either Benchmade or the Kershaw) and you get G10 handles. I love knives and in my opinion to find a sub $90 knife in S30V is a great deal. I think you should def get that one. As far as buying a second one. The Benchmade has a cult following But I think it is overpriced. I would pick a spyderco endura over the Kershaw for the same price due to the better steel, thumb ramp, and jimping. For the price of the benchmade, you can get a ton of great knives in the spyderco line up such as the Para military 2. Back a few years ago when Benchmade Osborne 942 were sub 130 bucks I would have said jump all over that; that knife is the fastest non-assisted manual opening knife I own. It is awesome. All in all, I would probably get the Benchmade grip if I had to pick from the 2. I think it is cooler and it has better steel.
edited to add...I totally forgot to say that a Spyderco Stretch in either vg-10 or zdp-189 would also beat out the benchmade or the kershaw. I have carried one for 6 years and its my favorite all time knife. It is great for edc and flexes nicely into lightweight tactical. It is also much faster for me than the endura.