Make sure your throttle endpoints (EPA) are set to their maximum; both forward and reverse. Then calibrate the ESC...
If you don't know how to set the EPAs, consult the manual. If you don't have the manual, check this one out:
http://www.spektrumrc.com/ProdInfo/F...30ManualHR.pdf
It's the manual for the Spektrum DX3.0 which is identical to the XS3. The manual for that radio was easier to find than the JR one