hey bigC,
Welcome to the site! I'd recommend a Mk4 VW TDI like my Jetta (yes, I'm biased), but unfortunately finding one in decent shape that is in the $3000 range would be very difficult. Having had a Honda hatchback (with which I've gotten some pretty good FE) and now having the TDI, I (personally) would never go back. The torque of the Diesel is awesome, and the ability to apply performance modifications which ALSO improve FE just seems too good to be true (and yet it is true!).
Of course just about anything you find will probably get better FE than the Jeep!
Oh! Just realized... I am looking to sell my VX in the next week or so... PM me if you think you might be interested.