  Frequency-domain multiplexing (fMux) is an established technique for the
readout of large arrays of transition edge sensor (TES) bolometers. Each TES in
a multiplexing module has a unique AC voltage bias that is selected by a
resonant filter. This scheme enables the operation and readout of multiple
bolometers on a single pair of wires, reducing thermal loading onto sub-Kelvin
stages. The current receiver on the South Pole Telescope, SPT-3G, uses a 68x
fMux system to operate its large-format camera of $\sim$16,000 TES bolometers.
We present here the successful implementation and performance of the SPT-3G
readout as measured on-sky. Characterization of the noise reveals a median
pair-differenced 1/f knee frequency of 33 mHz, indicating that low-frequency
noise in the readout will not limit SPT-3G's measurements of sky power on large
angular scales. Measurements also show that the median readout white noise
level in each of the SPT-3G observing bands is below the expectation for photon
noise, demonstrating that SPT-3G is operating in the photon-noise-dominated
regime.
