  This work presents a three-dimensional dynamical initialization model for
relativistic heavy-ion collisions, implementing local energy-momentum
conservation and baryon charge fluctuations at string junctions. Constraining
parameters using experimental data from p+p collisions at various collision
energies, the model provides a very good description of the charged hadron and
net proton rapidity distributions in Au+Au collisions from 7.7 to 200 GeV and
Pb+Pb collisions at 8.77 and 17.3 GeV. We demonstrate the importance of
fluctuations of baryon densities to string junctions for describing net-proton
distributions at collision energies of 62.4 and 200 GeV. Including this
improved baryon stopping description along with the requirement of strangeness
neutrality also yields a good description of identified particle yields as
functions of the collision energy above 7.7 GeV. We further study asymmetric
p+Al and (p, d, $^3$He)+Au collisions at the top RHIC energy and p+Pb, Xe+Xe,
and Pb+Pb collisions at LHC energies. We identify the produced particle
rapidity distributions in asymmetric collision systems as particularly useful
for constraining models of the early-time longitudinal dynamics.
