3" closed is pretty small... Here are some I can think of:
Spyderco Ladybug: 2.5" closed, 0.31" (8mm) thick, back lock
SOG Micron: 2.9" closed, 0.20" (5mm) thick, back lock
Buck 425 (MiniBuck): 2.9" closed, 0.31" (8mm) thick, back lock
Kershaw Chive: 3" closed, 0.28" (7mm) thick, frame lock, pocket clip
If you're willing to go a little over, the Dragonfly would be a great choice. The extra large choil allows you to choke up on the blade and get a decent 3-1/2 finger grip on the handle, giving it the feel of a slightly bigger knife. The others are about 2-1/2 to 3 finger grips (if you compress your fingers together on some). Some that are just over 3" closed:
SOG Flash 1: 3.25" closed, 0.35" (9mm) thick, arc lock, pocket clip
Spyderco Dragonfly2: 3.375" closed, 0.35" (9mm) thick, back lock, pocket clip
Kershaw Scallion: 3.5" closed, ?? thick, frame lock, pocket clip
Spyderco Gayle Bradley Air: 3.5" closed, ?? thick, liner lock
Here's a pic of most of the ones I mentioned. Unfortunately, I don't have a Scallion or Air

(), so I'm just listing the manufacturer specs. I included the Peanut and a Gerber Bear Grylls Compact Scout (similar in size to the LST) for comparison.