Of the 3, I would say Villa Galini. Firostephani is an easy 15-minute stroll to Fira town ... y ou can walk via the "rim path" going TO Fira (it's downhill) ... and coming back, there is a Flagstoned level path that is parallel to the main road ... it starts from the colonnade of a large church, it's between rows of private 1-2 story residences; ask your hoteliers to show you and/or mark it on the map.
The "beauty part" of Firostephani is that it's ignored by cruise crowds & bus-tour groups... they whizz right on by... so it's serene by day & night. To see OIA, there's a bus stop right on the main road; if you get the early bus (8 or 8:30) you'll have almost 2 hours to wander relatively uncrowded OIA lanes, and get some pictures in good morning light, before the throngs pour in, masses of confused looking people with nametags on, following guides with umbrellas or flags. When they show up, SCRAM.
OTHER SIGHTSEEING -- You can get a bus back to FIRA, and go to the small and GOOD museum, next to the Cathedral (the cruise crowds don't usually go there). If you're interested in the buried village of Akrotiri, you might google (or inquire here) about a guide; you'd probably have to pay for 2 hours if you meet the guide at the site -- could be €60 per hour. Alternative is to do some serious "homework" beforehand, because the signage is not great. NOTE: from Fira, you can easily take a bus to AKROTIRI ... & if you copy the timetable at the Fira "bus yard" you can time your stay to get a return bus ... it's also handy to have the cell phone # of a taxi service also. ANCIENT THIRA -- this is a great outing, doesn't really require a guide. Don't attempt to climb the whole thing; most time-efficient & cost-efficient would be to do a taxi to get there (at least to the high parking turn-around). From that point, it's about a 20-minute uphill scramble on a path, to the top. THere's a water fountain & a WC ... on a fine day, great place to have picnic with a view! Here's a good Map of ruins http://www.greeceprivatetours.com/ancient-thera-map.jpg and here's a Trip Advisor thread with me & some friends, https://www.tripadvisor.com/ShowTopic-g189433-i245-k3892417-Ancient_Thira_Kamari-Santorini_Cyclades_South_Aegean.html#28259953 with loads of tips, photos & links -- more about it than you want to know.