  A comprehensive study of three-photon electromagnetically-induced
transparency (EIT) and absorption (EIA) on the rubidium cascade $5S_{1/2}
\rightarrow 5P_{3/2}$ (laser wavelength 780~nm), $5P_{3/2} \rightarrow
5D_{5/2}$ (776~nm), and $5D_{5/2}\rightarrow 28F_{7/2}$ (1260~nm) is performed.
The 780-nm probe and 776-nm dressing beams are counter-aligned through a Rb
room-temperature vapor cell, and the 1260-nm coupler beam is co- or
counter-aligned with the probe beam. Several cases of EIT and EIA, measured
over a range of detunings of the 776-nm beam, are studied. The observed
phenomena are modeled by numerically solving the Lindblad equation, and the
results are interpreted in terms of the probe-beam absorption behavior of
velocity- and detuning-dependent dressed states. To explore the utility of
three-photon Rydberg EIA/EIT for microwave electric-field diagnostics, a
sub-THz field generated by a signal source and a frequency quadrupler is
applied to the Rb cell. The 100.633-GHz field resonantly drives the
$28F_{7/2}\leftrightarrow29D_{5/2}$ transition and causes Autler-Townes
splittings in the Rydberg EIA/EIT spectra, which are measured and employed to
characterize the performance of the microwave quadrupler.
