is newton's 3rd law of motion applicable in electrostatics?there is a statement that a uniformly charged shell does not exert any force on a charge placed inside it.now, we have a uniformly chargedshell and a charge placed inside it. will the charge exert any force on the shell and will the shell exert any force on the charge (since the field inside it is zero)?is the law violated in this case??

Yes, the Newton's third law is also applicable in electrostatics. Actually what happens is that each individual charge on the shell has an electrical field inside the shell and on applyin the superposition principle the sum of all the charges leads to the zero electrical field inside the shell. Now, the charge placed inside the shell does exert force on the charges on the shell but the sum on all the forces exerted by the charge is zero and hence that charge experience zero force in accordance with the Newton's 3rd law