  Rotating radio transients (RRATs) are peculiar astronomical objects whose
emission mechanism remains under investigation. In this paper, we present
observations of three RRATs, J1538+2345, J1854+0306 and J1913+1330, observed
with the Five-hundred-meter Aperture Spherical radio Telescope (FAST).
Specifically, we analyze the mean pulse profiles and temporal flux density
evolutions of the RRATs. Owing to the high sensitivity of FAST, the derived
burst rates of the three RRATs are higher than those in previous reports. RRAT
J1854+0306 exhibited a time-dynamic mean pulse profile, whereas RRAT J1913+1330
showed distinct radiation and nulling segments on its pulse intensity trains.
The mean pulse profile variation with frequency is also studied for RRAT
J1538+2345 and RRAT J1913+1330, and the profiles at different frequencies could
be well fitted with a cone-core model and a conal-beam model, respectively.
