Engion shakes at 20 mph, cel code 3 fast flashes, shaking goes away when foo is taken off gas pedal

Are you sure it's just 3 fast flashes?  It will repeat if you watch it.  Just asking because a single "3" doesn't sound accurate to any code I know.

 

http://legacycentral.org/library/literature/codes.htm

 

That's the list I use.