Re: Sorrento to Positano and Amalfi
There is a bus that travels from Sorrento to Amalfi and makes an automatic stop at Positano in the middle as well as a couple of other stops. Positano is the very popular stop so you cannot miss it.
There is also a ferry that travels from Sorrento to Amalfi and makes the same stop in Positano (tides permitting)
Sometimes even in nice weather the waves can be too high for the dock in Positano for the Sorrento boat to stop.
You can choose which direction to take which. Getting on the bus in Positano when very crowded in the summer can sometimes be an issue if the bus is full and not enough get off at Positano for you to get on. In that sense it is easier to take the bus from Sorrento or Amalfi. I don't think the ferry fills up the same way but busses come every 20 min. or so the ferry is no where near as frequent.
Positano is a vertical town. The ferry/dock is at the bottom the bus stop is at the top. It is better for your legs but also for the view when you get there I think to arrive by bus and depart by ferry.
Amalfi it does not matter the ferry and bus are near.
Sorrento the ferry requires some walking down to reach and then back up after so for most the bus in Sorrento is probably easier to reach from their lodging.
You don't have to book these things in advance so yes you can wait until you are there.
Given your time doing these 2 things is fine ; trying to do more would not be.
The area is in my opinion more beautiful than where you are staying otherwise (and I say that with some confidence not even knowing your plan) so that is the reason why I would say 4-6 nights for your stay in Sorrento/Amalfi Coast is better than your 3 nights.