Venice Ask your hotel for directions (including exact walking directions) from the airport. There are several ways, the best depends on where your hotel is. Stay on the island, not in Mestre. Some neighborhoods are more touristy, some are less expensive, all are good, you can everywhere on foot or by vaporetto (water bus). For the train you want to go from Venezia Santa Lucia to Firenze SMN (Santa Maria Novella).
Florence You can walk everywhere in the historic center from the train station. Florence is the best of your 3 cities for day trips. For the train you want to go from Firenze SMN to Roma (probably Termini, but usually the drop-down menus give you the option of "tutti le stazioni" all stations). It's probably best to take a taxi from the train to the hotel - it's easy to get lost in Rome. Most people choose Siena as a day trip, take the bus (about 1 hour), stops across the street from the train station. Pisa is a 1/2 day trip by train. Some recommend combining it with a 1/2 day in Lucca. I went to Pisa on my first trip to Italy and though it was about the least interesting place I saw, I was very glad I went. The campus is pleasant and the church and especially the baptistry are worth seeing.
Rome There's no central location, different neighborhoods have their own advantages and disadvantages. Buses, trams and the metro will get you everywhere you can't walk to, so best to choose a hotel near the metro and/or with several bus lines.
You can get by in English in Italy and in Paris. It's well worth the effort to learn simple polite phrases before you go. In stores, kiosks, cafes, everywhere, people expect you to say "good morning/afternoon/evening, sir/madam" first. Please, thank you, excuse me . . .