Two things shoot out in my mind right off:
We don’t know what the payload of the Shahab-3 is, so it is a guess whether the missile can actually carry a nuclear weapon that Iran could produce far enough to reach the target.
Also, just because you have the range, does NOT mean that you can drop it where you want. the fine course control needed in the final stage, just before impact is crucial if you want to hit your target. Otherwise you risk wasting that expensive missile and the nuclear weapon going off (IF it goes off!!) in the desert, missing the target by 100 miles and tipping your hand. It practically begs “NUKE ME!!!!”.
100 mile miss is a tad optomistic. Even a scud was more accurate than that. I would say they can hit close enough with their missiles, besides they have another type missile that has the same range as the Shahab-3. I am not sure what that is capable of.