I am spending 3 days down south and would appreciate some advice on a good place to base myself. Sorrento, Positano, Naples?

Depends what you want to do. If sight seeing or historic sights are of interest I would suggest Sorrento. It is easy to get to Capri, Naples, Pompei, Herculanem, Vesuvius. Going south a good trip is the Amalfi drive, spectacular views and you can stop at Positano and Ravello on the way.
I went to Pasteum (Greek ruins and museum) it was brilliant!
Paestum is a must if you like history also everyone goes to Pompei but I thought Herculaneum near by was as good if not better.
If you are near Naples visit the National Museum ,amazing! Here are some sites to look at which may give you ideas . I am going this summer and cant decide which area to go next!
italiantourism.com lonelyplanet.com/destinations/europe/italy/ roughguides.com

Cheers that is useful. I am keen to go to Pompei, Capri and Naples but would like to be out of the hustle and bustle. Is Sorrento a good base? What about Amalfi?

Sorrento is a good base because it has transport links to all of the places you mention. It's nothing special as a town though, so it depends how much you care about the place you stay. It's also cheaper than Positano or Capri.
Pompei and Herculaneum are both great, I personally preferred Pompei though. Positano, Amalfi and Capri (including the Grotto Azzuro) are definitely worth a visit! Ravello is also a great place to visit, it's a short bus ride up the mountain from Amalfi. There is a bus from Sorrento that goes to Amalfi via Positano, stopping at some other pretty places as well. Depending on the time of year Vezuvius is also worth a visit.

Excellent. Thanks for the advice. Should I book ahead for any of these tours, sights?

I might also suggest Pompei as a possible base. It is situated almost exactly between Naples and Sorrento on the commuter train line, and a hotel room in the middle of summer goes for around 90 euro (compared to 200+ for Sorrento or Positano). Plus, the town itself is quite "Italian", with a great piazza to hang out in at night in front of the Sanctuary (where miracles reportedly occur)and very few tourists after the Pompei Scavi close at night.
Naples is a bit sketchy in terms of hotels. Hard to find a good one, and the good ones tend to be rather pricy.
Positano and the Amalfi coast are gorgeous, but not super easy to base from, in my opinion.