MY cousin and I are cruising to Southhampton, UK with an overnight on our ship then traveling overnight to Portland. We would like to take a train to Portland,UK from Southhampton with a stop at a village or two to spend three days. We will have to board the ship on the third day. Any suggestions?
My first suggestion is to clearly communicate your plans to the cruise company and receive permission for this plan. You may well have already done so, but it not, it is a crucial step.
There is no train to Portland. It is a direct train from Southampton to Weymouth (a seaside resort) then a short bus ride across to the Isle of Portland.
You can't actually walk into the port complex at Portland. Your cruise company will bus you to the ship, presumably from Weymouth, but you will need to check that detail with them, You cannot even be dropped off by a taxi at the port gates but either in Weymouth or at Portland Castle for the port shuttle bus.
This recent thread may give you some ideas- https://community.ricksteves.com/travel-forum/england/train-from-bristol-to-wareham-dorset
Wareham, Swanage, the eastern Jurassic coast at Lulworth are all possible or even overshoot Weymouth and go to Lyme Regis.
Are you speaking of the Isle of Portland? Or are you meaning city of Portsmouth?
Also, still confused on your schedule, you say spend 3 days in a small village or two, but be on the ship the third day, which would be at most two nights someplace, but assume you want to spend some time in Portland/Portsmouth. Maybe a bit more detail on the itinerary would help with advice.