The first three-dimensional quantum-dynamical ab initio simulation of the large amplitude vibrational dynamics of an Ag-3 cluster initiated from its linear transition state is presented, This can be monitored by femtosecond pump-probe negative-ion to neutral-to-positive-ion spectroscopy with zero electron kinetic energy (NeNePo-ZEKE). The time evolution of the representative wavepacket confirms the dynamical effects which have been discovered previously by Wigner-type semiclassical simulations: the initial coherent bending and stretching from linear to near equilateral triangular configuration, followed by intracluster collision and intracluster vibrational relaxation with a weak fractional revival. In addition, the quantum wavepacket shows interferences and spreading.