multiple transmitters for one wireless headset

Hello,

I have a Sennheiser HDR140 headset and got 3 transmitter bases that work with the headset. each base have 3 channels for three different frequencies. what i am trying to do is power up all three transmitters, each at one channel transmitting an audio source (so 3 audio sources, 3 channels). also I put all the transmitters in one box. My problem is when I do that, two channels sound good while the third one is very static and basically a combination of the other two channels. here are the three channels' frequencies: Ch1 = 926.1-MHz, Ch2 = 926.8-MHz, Ch3 = 927.5-MHz.

thanks in advance!