  Simulations of relativistic heavy-ion collisions within the three-fluid model
employing a purely hadronic equation of state (EoS) and two versions of the EoS
involving deconfinement transition are presented. The latter are an EoS with
the first-order phase transition and that with a smooth crossover transition.
The model setup is described in detail. The analysis is performed in a wide
range of incident energies 2.7 GeV $< \sqrt{s_{NN}} <$ 39 GeV in terms of the
center-of-mass energy. Results on proton and net-proton rapidity distributions
are reported. Comparison with available data indicate certain preference of the
crossover EoS. It is found that predictions within deconfinement-transition
scenarios exhibit a "peak-dip-peak-dip" irregularity in the incident energy
dependence of the form of the net-proton rapidity distributions in central
collisions. This irregularity is a signal of deconfinement onset occurring in
the hot and dense stage of the nuclear collision.
