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Another option when I want to carry something a little more substantial (Endura, etc.) is to wear my Flip Belt (http://flipbelt.com/). I carry my phone in one side to listen to music (Bluetooth headphones) and run Endomundo and clip my Endura in the other side. Works like a champ and you never know its there. They never see the FLip Belt either because it's under my shirt.The problem with running shorts is that they are light weight and a knife with only minimal weight for regular shorts will have your running shorts sagging. If you must have a close to 3" blade a Syderco Salt 1 at 2 oz. is the best bet. The second choice, which is a bit slimmer top to bottom, is the Benchmade Lonewolf Mini Landslide at 2.22 oz. (also salt water rated). It is only $37 and was my choice, finding it easier to carry. Often, I will carry a simple FRN Dragonfly at 1.2 oz.
As a side note, the Mini Landslide has a stronger tip than the Salt 1. It comes in Orange, Green, and Pink.
The PM2 is pretty freakin light in my opinion. I've clipped it in my boxers/PJ/basketball shorts waistband many times. I think it would work as well as any others.
I'd look at the Spyderco Salt series - they are very light, with linerless FRN scales, and are very corrosion resistant, which could be helpful if you think you'll be sweating. I also like h1 generally, as it is easy to sharpen and holds a good enough edge. Only problem is that there is an endura-sized knife, and the dragonfly sized knife... neither are exactly the 3 inches you are looking for.
How about the Boker Urban Trapper? 3.5 inches of VG-10, titanium frame and under 2oz.
cncpro11, I'd forgotten about the BM 530 Pardue. That's an excellent suggestion.
I bought one for a co-worker when he retired, and I remember being shocked at how light it was.
That's a bunch of knife for under 2 ounces... 154CM, Axis Lock... dang you, now I need one.
While not a Spyderco The BM 530 with a coated blade is thin, light, has a nice grip to it and depending on needs and taste has either a plain of partially serrated blade.
I can clip it inside the waistband of my running shorts and never notice it while running. The black coated blade will inhibit corrosion.