To calculate the net electric force on each of the charges this is what you do:
For Part A: The force acting on charge 2 is as follows,
F2 = F12 + F32, (in other words the force acting on charge 2 is equal the force of charge 3 on 2, plus the force of charge 3 on 2)
After establishing this you go on to use the Electric Force Equation, which is:
F=(ke * q1*q2)/ r^2
where ke=8.988*10^9 N*m^2/C^2
and r is the distance between the two charges.
Do not forget to first convert the units of the charge and distance to the proper SI units of Coulomb and meter.
After you calculate F12 and F32, since they are vectors, you can find the magnitude by taking the square root of the sums squared, ie:
Magnitude = (X^2 + y^2)^(1/2)
and the direction can be determined using the formula: tangent(theta)= y/x
Be sure to conceptialize where the vector should be pointing because you will need to add 90 degrees to your answer until the angle is in the proper place.