A single-relaxation-time fluctuating lattice-Boltzmann (LB) model for direct numerical simulation (DNS) of particle Brownian motion is established by adding a fluctuating component to the lattice-Boltzmann equations (LBEs). The fluctuating term is proved to be the random stress tensor in fluctuating hydrodynamics by recovering Navier-Stokes equations from LBEs through a Chapman-Enskog expansion. A three-dimensional implementation of the model is also presented, along with simulations of a single spherical particle and 125 spherical particles at short times. Numerical results including the mean-square displacement, velocity autocorrelation function and self-diffusion coefficient of particles compare favorably with theoretical results and previous numerical results.