This video is pretty good in describing what Olympus was thinking with the new models.

https://www.youtube.com/watch?v=VnbP5YI5BCo

Here is another video with the same guy from Olympus talking to another magazine.  A little more detailed.

https://www.youtube.com/watch?v=ib_B--IBwRM

First big statement:

"All three will have the worlds fastest Auto Focus including all DSLRs"

"They all share the same sensor and image processor"

E-PM1 is the "Mini" target audience is people moving up from compact P&S who are intimidated by too many buttons and dials.  Easy to use UI mainly. And it is actually smaller than the other models.  Attachable flash is included and has a bounce function.

E-PL3 is the "Light".  Intermediate target audience.  Has tilt screen and all the manual button controls.  Same flash as E-PM1 included.  Save a little money over the E-P3.

E-P3 is the "Pen".  "It's a real top end high end camera".  Biggest difference over E-PL3 is the touch screen OLED.  Built in flash. 

Good news is that all of the cameras will take the same quality pictures.  Your just paying more or less for different ergonomics.  All of the bodies are aluminum, but the E-P3 has the distinctive steel wrap.