For part (a) I wonder if you were expected to do simple electrostatics and geometry--adding up the single-particle fields, and doing first-order calculations for the distances. (I note that in your derivation you didn't insert the unit vectors until the last steps, so technically you're equating vectors to scalars.)
For (b) I don't know what they mean by a "singular" charge density, but perhaps they're thinking of something like p-orbitals.
For (c) I think this is definitely a case of simple electrostatics and geometry. Define a uniform electric field along a general direction (by symmetry this just means defining one angle) and work out the force on each charge separately; add up the results. The torque will be related to the difference in forces on the three charges (actually only the outer two are important).