I mean we are not talking about charging the vehicle to full. You only need like a hand portable battery to jump start the low voltage system.
The procedure is described in the owner's manual: "In the unlikely event that Model 3 loses electrical power, you cannot access the touchscreen and are therefore unable to release the parking brake without first jump starting (see Jump Starting)."[1]
And the jump starting procedure is described here: https://www.tesla.com/ownersmanual/model3/en_au/GUID-3567D5F...
1: https://www.tesla.com/ownersmanual/model3/en_au/GUID-3DFFB07...