Day trip to Amalfi Coast

That sounds lovely, but it will make an awfully long day. Maybe split it into two day trips, or eliminate at least one of the towns?

I don't remember how often the ferries run, but that needs to be added into your calculations, as well.

If it were me, (and it isn't, I know) I'd skip Positano. Amalfi and Ravello are much more interesting, in my opinion. I'm sure you'll get lots of suggestions here, though.

Excellent plan! It's what I do every year and have never regretted it. Except one small change - sometimes I take the bus back from Amalfi. It starts there so I can get a seat on the driver's side and pretty much get the views. But the boat ride is still my favorite. Last year there was a package that stopped in Amalfi for a few hours then stopped in Positano for a few hours. On the way back to Sorrento, the boat hugged the coast for some amazing views. Several years ago friends were with me - we found a boat trip (same itinerary) that also stopped along the coast of Capri where everybody jumped ship and swam around for about an hour. We're still (10 years?) talking about that!

Ferry schedule and routes here

https://www.positano.com/en/ferry-schedule?from=sorrento&to=positano#

Great plan but will be a long day. I’d actually skip Positano, it’s prettier from the water, Not much IN town other than shopping
Go right to Amalfi
Ravello is lovely, walk out to Villa Cimbrone for the fabulous views and gardens

Do try to take bus back to Sorrento from Amalfi as Robert has suggested

Here's what I'd do.

Ferry Sorrento to Amalfi bus immediately to Ravello for a visit. Enjoy lunch here. Then bus back to Amalfi to explore. Ferry to Positano having dinner here then bus back to Sorrento.

Check return bus times carefully to Sorrento. If you miss the last one, it's an expensive taxi trip.

https://www.positano.com/en/bus-schedule?timetable_route=amalfi_positano_sorrento
This schedule will change for summer travel, but don't plan on the last bus.

gerri's plan is perfect. Do that.

There is a ferry that goes from Sorrento to Positano to Amalfi and reverse but probably won't give you enough time there. So you can take the ferry there and the bus back. In fact I prefer to do boat one way and bus the other as the scenery is different. So gerri's plan or some version of it is good.

One other option to see Amalfi Coast in a day: My wife and I stayed in Sorrento for 3 nights in Aug 2019. As part of that we did a one-day trip to Amalfi, using the ""Shared Tours Mondo Guide for Rick Steves". Friendly transportation to 3 villages (Positano, Amalfi, Ravello) with free time at each village. I posted a review in the forum:
https://community.ricksteves.com/travel-forum/italy/amalfi-coast-excellent-value-shared-tours-mondo-guide-for-rick-steves

You may end up waiting to get on the Ravello bus in Amalfi. You won't be the only ones wanting to take it. That may even mean waiting for a second bus (30 minutes?). Maybe consider a taxi?