Day Trip from Sorrento

The only suggestion I have is to NOT try to do all 3 in one day. 2 are doable without totally exhausting yourselves or just skimming over the main highlights of Pompeii. Even the main sights of Pompeii take a good 4 hours.

Sorrento is an ideal place to do this from, but as noted, it's too much for one day. Month of the year is VERY important, because of SUN, HEAT, and crowding. Do you understand that Vesuvius is a big, steep, exposed heap of ash with little to see? I didn't bother, and I've never regretted it.

Pompeii and Herculaneum are both on the Circumvesuviana train, but Vesuvius requires a bus, taxi, or jitney ride. Most people take a commercial mini-bus near the Pompeii Porta Marina entrance. There are multiple companies. You might inquire how far to the top it is from where the bus will drop you, and how long you have at Vesuvius.

As stated, Pompeii is an easy trip by train from Sorrento. Note that your station is Pompeii Scavi, not Pompeii, which is in town. The station is almost right at the Pompeii gate.

We did Pompeii by train from Sorrento. The site is huge and we were not able to see to all in one day. We returned the next day to do Herculaneum. That would be a good time to add Vesuvius.
If you are a fan of Greek temples you might want to consider doing another day trip to Paestum. It requires a charter tour and a long day on the bus but it is an outstanding site.
Here is what we saw in our week in Sorrento.

Sorrento http://www.flickr.com/photos/stanbr54/sets/72157634654031558/

It requires a charter tour and a long day on the bus but it is an outstanding site.<<<

Not necessarily.

We stayed 2 nights in Salerno after our time in Sorrento/Amalfi.
Very easy short train ride to Paestum from there, and yes it is a wonderful sight- well worth the extra travel/time.
Or one could probably take bus to Amalfi- then ferry to Salerno then train to Paestum as long as timing works out- the train from Salerno to Paestum has a mid day lull, so you'd have to check schedules carefully.
Salerno is also a delightful town to visit.

But remember that (in MAY !) every bus to Amalfi we saw originating in Sorrento had a line that was more people than would fit on the next Half-Hourly bus. When is a bargain not a bargain? When it uses up your limited time.

Edit: It's also important to know that Herculaneum is a 15-minute, steep downhill walk from the Circumvesuviana station. There is a big cafeteria-style restaurant in Pompeii, but no food inside Herculaneum. You cannot exit for food and re-enter with a same-day ticket to either ancient site.

Since the OP was asking about day trips from Sorrento I mentioned the guided bus tour we took from there. Having said that, this tour included an hour and a half stop in Salerno which we enjoyed.

Depending on the season, Herculaneum is open late (sunset essentially.) This makes doing both Pompeii and Herculaneum in one day very doable. Get an early start at Pompeii, take a nice long lunch siesta break and see Herculaneum in the late afternoon/early eve. You’ll avoid the midday heat, and more importantly the crowds. I do recommend both sights equally. They are very different and rewarding for their own reasons.