The BattleMeshV3 will take place along the border of the 'lago di Bracciano' -- a lake just north-west of Rome.
How to get there by public transportation
From the airport of Ciampino:
take a shuttle bus to the Termini station, then take the metro A to the Valle Aurelia station. There take the train (line FR3 direction Viterbo) to Bracciano.
OR take a bus to the Anagnina metro station (cheaper), then take the metro A and get off at the Valle Aurelia station. There take the train (line FR3 direction Viterbo) to Bracciano.
From the airport of Fiumicino: take the train to Rome and get off at the Roma Ostiense (or Roma Trastevere) station, then take another train (line FR3 direction Viterbo) to Bracciano.
From the Termini central station: take the metro A to the Valle Aurelia station. There take the train (line FR3 direction Viterbo) to Bracciano.
Notes:
there are two railway lines that end at Viterbo: the "Roma-Civita Castellana-Viterbo" line, and the FR3 line. Take the FR3 line.
- some trains of the FR3 line have their last stop at the Cesano station, before Bracciano. In case you take one of these trains, just wait for the next one going to Bracciano.
A map with the metro and city railways can be found here: metroandrailways.pdf
Timetables and other info on the Italian railways website, and on Rome's public transportation website.