Four particles, cach carrying a charge of magnitude \(3.0 \mathrm{nC}\), are at the corners of a square that has a side length of \(3.0 \mathrm{~m}\). For zero potential set at infinity, calculate the potential at the center of the square \((a)\) if all particles are positively charged, \((b)\) if three particles are positively charged and one is negatively charged, and \((c)\) if two particles are positively charged and two are negatively charged.