In a region of space three charged particles observed to be in equilibrium under their mutual electrostatic forces only. Then the particles

Three charged particles are in equilibrium under their electrostatic forces only when the particles are on the same line (i.e., collinear) so that the forces exerted  on a particle by the other two paticles are equal in magnitude and oppositely directed. Two of the charges must be of same sign and the third must be of opposite sign.Also since the separation between the charges are different , the charges cannot have the same magnitude . Apart from this, the equilibrium is unstable because if the central charge is slightly displaced along the line joining the charges to one side and subsequently released , it will never return to its initial position.