  We present an analysis of the merging cluster MACS J1149.5+2223 using
archival imaging from Subaru/Suprime-Cam and multi-object spectroscopy from
Keck/DEIMOS and Gemini/GMOS. We employ two and three dimensional substructure
tests and determine that MACS J1149.5+2223 is composed of two separate mergers
between three subclusters occurring $\sim$1 Gyr apart. The primary merger gives
rise to elongated X-ray morphology and a radio relic in the southeast. The
brightest cluster galaxy is a member of the northern subcluster of the primary
merger. This subcluster is very massive
(16.7$^{+\text{1.25}}_{-\text{1.60}}\times\text{10}^{\text{14}}$ M$_{\odot}$).
The southern subcluster is also very massive
(10.8$^{+\text{3.37}}_{-\text{3.54}}\times\text{10}^{\text{14}}$ M$_{\odot}$),
yet it lacks an associated X-ray surface brightness peak, and it has been
unidentified previously despite the detailed study of this \emph{Frontier
Field} cluster. A secondary merger is occurring in the north along the line of
sight with a third, less massive, subcluster
(1.20$^{+\text{0.19}}_{-\text{0.34}}\times\text{10}^{\text{14}}$ M$_{\odot}$).
We perform a Monte Carlo dynamical analysis on the main merger and estimate a
collision speed at pericenter of 2770$^{+\text{610}}_{-\text{310}}$ km
s$^{-\text{1}}$. We show the merger to be returning from apocenter with core
passage occurring 1.16$^{+\text{0.50}}_{-\text{0.25}}$ Gyr before the observed
state. We identify the line of sight merging subcluster in a strong lensing
analysis in the literature and show that it is likely bound to MACS J1149
despite having reached an extreme collision velocity of $\sim$4000 km
s$^{-\text{1}}$.
