Remote Controlled Vehicle Design
If you control motor speed by varying the amplitude of the sine wave then your filters will have to be very sharp to reject unwanted frequencies at higher amplitudes. 

Instead, I would treat each frequency as a digital (on/off) channel, and apply PWM (Pulse Width Modulation) to control the speed. If each channel is either on or off then its filter only has to be sufficiently sharp to reduce the amplitude of the other channels enough to reliably detect the difference. 

If the frequencies are always sent at full amplitude then you can detect the signal with a simple comparator at the output of each filter, set to eg. 50% amplitude. Anything below the threshold then means that the channel is off, whereas if you used amplitude to control motor speed you wouldn't know if it was a low throttle signal or another channel 'breaking through' the filter. 

The filters will slow down the receiver's response to amplitude changes and prevent short pulses from getting through. So to avoid distortion the PWM frequency should be much lower than the 'carrier' frequency. However a high PWM frequency is desired to make the motors run smoothly and efficiently. 

Therefore I would use the lower 10KHz and 30KHz channels for motor selection (one channel per motor and buttons for left channel only, right channel only, and both together) and the higher 50KHz channel for PWM. A PWM frequency of 500Hz should be high enough to run the motors smoothly, yet still 100 times lower than the carrier frequency to minimize pulse width distortion.   

This tone pulsing technique was used in early model radio control sets. These were commonly known as 'reed' systems because they used an electromechanical detector that looked a bit like a harmonica, with reeds energized by the audio signal and tuned to different frequencies. A screw above each reed could be adjusted to make contact with the reed when it vibrated with sufficient amplitude. Your circuit would be reproducing the functionality of this 1950's technology!