  We present new high contrast imaging of 8 L/T transition brown dwarfs using
the NIRC2 camera on the Keck II telescope. One of our targets, the T3.5 dwarf
2MASS J08381155 + 1511155, was resolved into a hierarchal triple with projected
separations of 2.5+/-0.5 AU and 27+/-5 AU for the BC and A(BC) components
respectively. Resolved OSIRIS spectroscopy of the A(BC) components confirm that
all system members are T dwarfs. The system therefore constitutes the first
triple T-dwarf system ever reported. Using resolved photometry to model the
integrated-light spectrum, we infer spectral types of T3, T3, and T4.5 for the
A, B, and C components respectively. The uniformly brighter primary has a bluer
J-Ks color than the next faintest component, which may reflect a sensitive
dependence of the L/T transition temperature on gravity, or alternatively
divergent cloud properties amongst components. Relying on empirical trends and
evolutionary models we infer a total system mass of 0.034-0.104 Msun for the BC
components at ages of 0.3-3 Gyr, which would imply a period of 12-21 yr
assuming the system semi-major axis to be similar to its projection. We also
infer differences in effective temperatures and surface gravities between
components of no more than ~150 K and ~0.1 dex. Given the similar physical
properties of the components, the 2M0838+15 system provides a controlled sample
for constraining the relative roles of effective temperature, surface gravity,
and dust clouds in the poorly understood L/T transition regime. Combining our
imaging survey results with previous work we find an observed binary fraction
of 4/18 or 22_{-8}^{+10}% for unresolved spectral types of L9-T4 at separations
>~0.1 arcsec. This translates into a volume-corrected frequency of
13^{-6}_{+7}%, which is similar to values of ~9-12% reported outside the
transition. (ABRIDGED)

Solar and Stellar Astrophysics