  We present the results from a detailed ASCA/ROSAT X-ray study of three
distant, massive cooling flows; Zwicky 3146 (z=0.291), Abell 1835 (z=0.252) and
E1455+223 (Zwicky 7160; z=0.258). Using multiphase models fitted to the ASCA
spectra, we determine values for the temperature, metallicity, luminosity and
cooling rates in the clusters. These results are combined with deprojection
analyses of the ROSAT images to provide detailed constraints on the mass
distributions in the systems, and on the properties of their cooling flows. The
spectral and imaging data identify these clusters as the three most-massive
cooling flows known, with mass deposition rates of ~ 1400, 2300, and 1500 solar
masses per year, respectively. We highlight the need for multiphase models to
consistently model the spectral and imaging X-ray data, and discuss the
importance of using these models in X-ray determinations of the cluster masses.
We also present results from an extensive optical study of the clusters and
report the discovery of a gravitational arc in Abell 1835. The lensing data
provide an indepenent constraint on the distribution of mass in the cluster, in
good agreement with the results from the mutliphase (although not single-phase)
X-ray analysis. We present measurements of the galaxy distributions in Abell
1835 and E1455+223 and relate these to the distributions of the total mass. The
ASCA spectra place firm constraints on the column density of intrinsic X-ray
absorbing material in the clusters. Abell 1835 and E1455+223 exhibit large
intrinsic column densities associated with their cooling flows. These clusters
also exhibit significant amounts of reddening in the optical spectra of their
central galaxies. The data for Zwicky 3146 indicate lower lower levels of X-ray
absorption and reddening. All three clusters exhibit excellent alignment
