EDIT: Note this is for the first version of the DX3.0, I don’t know if the latest version is the same or not.

Press the Scroll button until you see a small EPA in the screen (End Point Adjustment).
Press the Channel button until you see TH (Throttle).
Pull the trigger for throttle if needed to get a L/F (Left/Forward).
Adjust with the increase and decrease buttons. Normally to 100.
Push trigger and do same for brake (or reverse). R/B, which I suspect is Right/Brake.

As mentioned it’s important to calibrate the speedo to the radio, setting the neutral/full throttle/full brake (or reverse).

If it’s still as described and it’s a sensorless brushless (or brushed) setup it’s possible that the motor wiring is backwards, and thus forward/reverse is backwards. Flip any 2 wires and recalibrate the speedo. (Don’t try that if it’s a sensored deal!) Might need to use servo reversing on the radio.