Laser pulses of moderate intensities with durations of either 1.5 ps or 120 fs were employed to excite the Na-3 molecule to its electronic B state. Using a pump-probe technique the temporal evolution of the two-photon ionization signal could be resolved in real time. Different vibrational modes of the excited trimer are detected selectively, While the ps laser pulses yield preferential excitation of the slow pseudorotational mode with a period of 3 ps, the use of similar to 10 times shorter pulses allows the trimer’s symmetric stretch mode with a 310-320 fs period for the first 5 ps to be observed. These complementary experimental results can be explained to a great extent by quantum dynamical simulations of the pump-probe experiments. The calculations are performed on three-dimensional ab initio potential energy and transition dipole surfaces. Thus all three vibrational degrees of freedom of the Na-3 molecule are included in the theoretical treatment. The time-dependent wave-packet dynamics elucidate the effect of ultrafast state preparation on the molecular dynamics. Extensive theoretical calculations manifest the possibility of initiating the molecular dynamics dominantly in selected modes during a certain time span by variation of the pump-pulse duration.