  We present a multi-wavelength study of the massive ($M_{200\textrm{c}}
\approx 1$-$2 \times 10^{15} M_\odot$) galaxy clusters RXC J2248.7$-$4431, MACS
J0416.1$-$2403, and MACS J1206.2$-$0847 at $z \approx 0.4$. Using the X-ray
surface brightness of the clusters from deep Chandra data to model their hot
gas, we are able to disentangle this mass term from the diffuse dark matter in
our new strong-lensing analysis, with approximately $50$-$100$ secure multiple
images per cluster, effectively separating the collisional and collisionless
mass components of the clusters. At a radial distance of $10\%$ of
$R_{200\textrm{c}}$ (approximately $200$ kpc), we measure a projected total
mass of $(0.129 \pm 0.001)$, $(0.131 \pm 0.001)$ and $(0.137 \pm 0.001)\times
M_{200\textrm{c}}$, for RXC J2248, MACS J0416 and MACS J1206, respectively.
These values are surprisingly similar, considering the large differences in the
merging configurations, and, as a consequence, in the mass models of the
clusters. Interestingly, at the same radii, the hot gas over total mass
fractions differ substantially, ranging from $0.082 \pm 0.001$ to $0.133 \pm
0.001$, reflecting the various dynamical states of the clusters. Moreover, we
do not find a statistically significant offset between the positions of the
peak of the diffuse dark matter component and of the BCG in the more complex
clusters of the sample. We extend to this sample of clusters previous findings
of a number of massive sub-halos higher than in numerical simulations. These
results highlight the importance of a proper separation of the different mass
components to study in detail the properties of dark matter in galaxy clusters.
