Kh and pH are linked. As carbonate hardness increases, alkalinity increases, so pH has to increase as well.
This is how we're able to use the Kh/pH chart to determine our CO2 levels....without a strong phosphate buffer, Kh & pH should be proportionate to each other. Adding CO2 creates carbonic acid which lowers pH, but leaves Kh untouched...this skewing of the Kh/pH relationship is how we see that a Kh of 8 degrees and a pH of 7.0 yields a CO2 level of roughly 25ppm - ideal level of CO2.
That's whats strange about your water...3d Kh is the bare minimum recommened Kh level if you're going to inject CO2. At 3dKh I'd expect a tap pH of only 7.4 (my Kh is 8degrees, pH 7.8)
I suggest you test your tap water after it has sat out for 24 hours in a bucket with an airstone running in it.
Onyx would increase your pH and Kh, but only by about .2 and 1 degree respectively.