Calculate Buffer Op Amp Output Impedance (With Data Sheet)
When the amplifier is operated with the "loop closed" i.e. it is run at (say) a gain of unity or ten or other normal sorts of gains, then the effective output impedance drops massively from 3 kohm to sub 1 ohm at DC.

This is due to the effect of negative feedback. So, you look at the open loop gain (about 120 dB or 1 million at dc) and factor this and the circuits target closed loop gain to calculate closed loop output impedance.

If your target closed loop gain were ten then the DC output impedance would be about 3 kohm / 100,000 = 30 milli ohm

At 100 kHz the open loop output impedance is about 400 ohms but the open loop gain is now reduced to 40 dB (100) so now the closed loop output impedance is about 40 ohms (assuming the closed loop gain is ten). 

If the closed loop gain were unity then the closed loop output impedance at 100 kHz would be 4 ohms.

One more thing to remember is that the open loop output impedance is a real measure of the output transistor's ability to deliver current to a given output load so consider this: If the DC output impedance is 3 kohm and the load is 1 kohm, the maximum peak dc voltage that can be produced at the output is severely limited by this potential divider irrespective of the chosen closed loop gain. 

Personally, I think that TI are potentially pulling the wool over people's eyes given that they state in the DS (page 8) that the maximum volt drop with a 2 kohm load (to mid rail) is only 500 mV. They refer to this as: -


  Voltage output swing from rail


And, it can only be for high frequencies else it contradicts the open-loop output impedance graph. Be aware of this.