What % is it stuck at?
It's been a while since I had this radio, but I remember running into a similar problem. Keep in mind that dual rate is only a percentage of the EPA. Try this and see if it works; set your dual rate to 70% (the default value). Then set your EPA. Now see if you have the flexability you want.
If your EPA is too high it might make sence that your dual rate is not going as low as you would like.
From the DX3.0 manual...
The Steering Dual Rate Trim Adjustment, located at Grip Lever B, allows the dual rate value (maximum servo travel) to be increased or decreased within a range from 100% through 20% of the total end-point value established in the steering EPA function. This function is very useful in race conditions as it allows you to custom tailor the steering radius and sensitivity for the current track conditions. Please note that since the Dual Rate value shown in the “STG” screen is the percentage of the end-point value established in the steering EPA function, the value will not always increase or decrease each time the Grip Lever B is moved.
You might also need to make a mechanical adjustment to the servo horn, but I think your problem can be fixed with the TX settings. I think your EPA is probably not set right.