In this paper, a range based localization method is proposed for position estimation in three dimensions with only three measurements. The proposed scheme is based on a modification of the system to make it globally observable and a change of coordinates which allows to construct a continuous time observer. The obtained continuous time observer is further modified to be able to deal with aperiodic and asynchronous measurements. The performances of the proposed approach are illustrated with simulations. It is shown that the proposed observer performs well even in presence of noise.