  Rotationally-modulated variability of brown dwarfs and giant planets provides
unique information about their surface brightness inhomogeneities, atmospheric
circulation, cloud evolution, vertical atmospheric structure, and rotational
angular momentum. We report results from Hubble Space Telescope/Wide Field
Camera 3 near-infrared time-series spectroscopic observations of three
companions with masses in or near the planetary regime: VHS J125601.92-125723.9
b, GSC 6214-210 B, and ROXs 42 B b. VHS J1256-1257 b exhibits strong total
intensity and spectral variability with a brightness difference of 19.3%
between 1.1-1.7 $\mu$m over 8.5 hours and even higher variability at the 24.7%
level at 1.27 $\mu$m. The light curve of VHS J1256-1257 b continues to rise at
the end of the observing sequence so these values represent lower limits on the
full variability amplitude at this epoch. This observed variability rivals (and
may surpass) the most variable brown dwarf currently known, 2MASS
J21392676+0220226. The implied rotation period of VHS J1256-1257 b is
$\approx$21-24 hr assuming sinusoidal modulations, which is unusually long for
substellar objects. No significant variability is evident in the light curves
of GSC 6214-210 B ($<$1.2%) and ROXs 42 B b ($<$15.6%). With a spectral type of
L7, an especially red spectrum, and a young age, VHS J1256-1257 b reinforces
emerging patterns between high variability amplitude, low surface gravity, and
evolutionary phase near the L/T transition.
