We will be travelling to Venice from Lake Como by car and staying in Venice for 3 nights before moving on to Tuscany. Parking at the Piazzale Roma is expensive. What is the safest place to park on the mainland if we will be leaving the car there for 3 days?
Your question is a bit bipolar; you say parking at Piazzale Roma is expensive but then
you ask for a safe place to park.
Obviously you are also going to need a parking place which is accessible to xport
to get into Venice. So someplace in Mestre seems like the best choice.
I found you this place which seems to tick all the necessary boxes, although I will
say I haven't parked there: https://www.venicecitypark.com/.
I parked at SABA at the Mestre station some years ago.
https://www.sabait.it/it/parcheggio-venezia-mestre/parcheggio-saba-stazione
I see they now charge 22€ a day.
Otherwise check this website and compare.
https://www.myparking.eu/parcheggio_stazione_venezia_mestre.php?gad_source=1&gbraid=0AAAAAD-pHxSCnq9lfqJStniABGoYhGvkK&gclid=EAIaIQobChMI6djs3LWPhwMV0ZmDBx03NQIwEAAYASAAEgJVHfD_BwE
There are also free lots but they are way far from the station and obviously totally unguarded.