On my spectrum DX6i there is an option to do a range check where the power of the transmitter is reduced and you should still be able to control the receiver at a few meters distance.
Does the DX3 have the same option?
If your having problems with two receivers then the problem is likely the transmitter or the receiver installation.
In a car its best to have the receiver aerial pointing vertically. I have read on other forums that large metal objects (like helicopter engines) can block signals to receivers. For this reason some aircraft have satellite receivers that can be placed at multiple positions around the model.
It might be worth moving the receiver to a different position, or even temporarily taping it to the very top of the car to see if different positioning helps.
Horizons tech guys are very helpful if it does turn out that something is faulty.
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