Short Coax for carrying low frequencies (120kHz) - does it matter if 50 Ohm or 75 Ohm?
I'm using coax to carry signals whose highest frequency component is
  120kHz


Consider the impedance of coax or twisted pair in the range DC to 1 MHz: -



At about 100 kHz or higher, the cable impedance is going to be constant i.e. 50 ohms or 75 ohms but, if your signals range significantly down towards DC then the cable impedance begins to be dominated by R and Xc. So, at 1 kHz the impedance might be about 300 ohms and, at 10 Hz, the impedance might be as high as 3000 ohms.

So, I wouldn't particularly worry about it because the cable impedance is not constant and the issue of 50 ohms versus 75 ohms has flown out of the window. Also, at only 2 or 3 metres, there won't be any reflections of note to worry about because the signal wavelength is massively longer than the cable length.