I'm not clear from your description what you are doing.
I think you mean you are afraid they will interfere with each other.
Do you mean 8 cables or 8 wires? RJ11 is 6P, so up to 6 wires.
You can't put a cat 5 cable into an RJ11 connector, RJ11 is 6P, a cat 5 cable is 8P so won't fit, or did you mean use cat 5 cable with an RJ11 plug and the other wires loose?
You can't just pick wires at random, they are in twisted pairs and you have to respect that. I suggest 1 wire of the pair for signal, the other grounded.
I2C is not designed for more than a short distance, however it can be made to work over much longer distances than 3m, I have it working over about 30m without any problems. Set the clock speed low, use low value pull up resistors (maybe 1k) and use twisted pairs. Over 3m I would expect it to work OK over flat telephone cable, but I've not tried.