Hmm, yes I took a look at the ESE Labs website pages on the Nibiru, and it indeed specifies 3 ohms. It seems to be oriented towards extremely low output moving coil cartridges, which would presumably have very few turns in the coils, and therefore very low resistance and inductance, which would in turn improve compatibility with heavy (low-value) loading.
The unusually low input impedance seems to derive from what they describe as a current sink/current domain/transimpedance design approach. I have no idea how to reconcile that, though, with their statement that it is suitable for cartridges of up to 40 ohms impedance, if severe volume attenuation is to be avoided. And such cartridges are also likely to have considerable coil inductance, resulting in the severe treble rolloff that I mentioned, in combination with the severe loading.
You might find the following to be of interest:http://www.hagtech.com/loading.html