The NLTE Fe I line formation problem in a three-dimensional granulation model of the metal-poor halo star HD 140283 is considered. The self-consistent solutions of the kinetic and radiative transfer equations were obtained with a realistic atomic model Fe I + Fe II. The effects of horizontal radiative transfer are neglected. The near-UV overionisation effect is found to lead to sizeable Fe I opacity deficits in the granular atmospheric regions. This NLTE effect tends to produce weaker emergent spectral lines than in LTE because they are formed deeper in the stellar granules where the source functions are close to the Planck function. The NLTE effects in the integranule spectra are small. If NLTE effects are shown to be fully taken into account both in the three-dimensional and one-dimensional models of the star HD 140283 the iron abundance is in close agreement for both cases.