I visited trenitalia.com and am a bit confused

will be traveling via rail from:
rome to florence
florence to venice
venice to rome

there seem to be multiple ending stations for each train...looking for the most central (to downtown) for each city.

seems like rome= termini
florence= santa maria novella
venice= santa lucia

is this true?

yes Santa Lucia for Venice (not Mestre)

Also Termini for Rome and Santa Maria Novella for Florence.

Venezia Santa Lucia, Firenze Santa Maria Novella, and Roma Termini are the main train stations in their respective cities. Venice has one other train station (Mestre), Florence has several, and Rome has a truckload (Tiburtina and Ostiense being two medium-tier train stations).

Also learn the acronyms:
VE[nezia] SL = Santa Lucia
FI[renze] SMN = Santa Maria Novella
Roma TE = Roma Termini

ttt

Are you returning to Rome for your return flight? If so, it might be easier to depart from the Venice airport.

ttt