  We demonstrate Rydberg atom-based radio frequency sensing with a colinear
three-photon scheme in a room temperature cesium vapor cell that minimizes
residual Doppler broadening of the probe laser absorption feature. A sub-200
kHz spectral linewidth is observed and extends the self-calibrated
Autler-Townes sensing regime to weaker fields by a factor of ~18 compared to
the theoretical limit of the most commonly used two-photon scheme. The
sensitivity of the method to microsecond pulses is shown to be sufficient to
detect radio frequency Rabi frequencies of 2$\pi$ x 0.44 MHz using a 480 kHz
bandwidth at 108.9 GHz, demonstrating the ability to sense time-dependent
signals suitable for radar and communications.
