Once you have set up the expo in the DX3R it will remain that way it can't be changed, but there is a switch you can use to adjust the sensitivity or EPA of the brake, this will also affect the throttle on the low end, a throttle curve will fix the problem as I have the same problem too, but the DX3R does not has one, it is really the only thing I don;t like of the radio, I placed a preorder for the new one anyway.

I'm trying to see how will the DX3S solve this, as they have a new function called throttle Punch, tried to download the manual but there is an error in their system.

On the mean time you can use either the expo, or move the servo arm away from the regular center and engine, this will give you some mechanical expo in the bottom only, but you have to readjust the EPA and the brakes.

Hope this helps.