It's probably possible to build a 3" flying wing...
For that size, I'd snag the smallest direct drive motor you can, actually, you can probably just use this:
Use an IPS or LPS motor from GWS to power your plane. Just use that motor, a two cell li-poly from 250-800mah's, a light Rx (R4PII or less), some .20 or .16 oz servos, and a two amp esc. Should be a piece of cake.
I'd bet the LPS setup is your best. Balsa products had a LPS short brushless motor + ESC combo, but they're out of stock. You may want to use the cirrus joule servos and Rx + ESC's to stay on the safe side. It's just that they get expensive.