An electrostatic trapping scheme using three charged spherical electrodes with opened optical access for cold polar molecules in the weak-seeking states is proposed, and the schematic drawing is given. The analytical expressions for the space electrical field distribution are derived with the image charge method. The numerical solution of electrical field distribution is calculated by the finite element software. Dynamic process of the loading and trapping cold polar molecules in an electrostatic trap is simulated with the classical Monte Carlo method. The influences of velocity of the incident molecular beam and loading time on the loading efficiency are studied, and the temperature of the trapped cold molecules is given. Some potential applications of the scheme in the electrostatic surface trapping especially the electrostatic lattices for cold polar molecules are discussed. Results show that the loading efficiency can reach 47.4%, and the temperature of trapped cold molecules is 25.4 mK.