  We model the evaporation histories of the three planets around K2-136, a
K-dwarf in the Hyades open cluster with an age of 700 Myr. The star hosts three
transiting planets, with radii of 1.0, 3.0 and 1.5 Earth radii, where the
middle planet lies above the radius-period valley and the inner and outer
planets are below. We use an XMM-Newton observation to measure the XUV
radiation environment of the planets, finding that the X-ray activity of K2-136
is lower than predicted by models but typical of similar Hyades members. We
estimate the internal structure of each planet, and model their evaporation
histories using a range of structure and atmospheric escape formulations. While
the precise X-ray irradiation history of the system may be uncertain, we
exploit the fact that the three planets must have shared the same history. We
find that the Earth-sized K2-136b is most likely rocky, with any primordial
gaseous envelope being lost within a few Myr. The sub-Neptune, K2-136c, has an
envelope contributing 1-1.7% of its mass that is stable against evaporation
thanks to the high mass of its rocky core, whilst the super-Earth, K2-136d,
must have a mass at the upper end of the allowed range in order to retain any
of its envelope. Our results are consistent with all three planets beginning as
sub-Neptunes that have since been sculpted by atmospheric evaporation to their
current states, stripping the envelope from planet b and removing most from
planet d whilst preserving planet c above the radius-period valley.
