Just use Zener diodes for overvoltage protection, P-MOSFET on input for reverse voltage protection and some PTCs or even fast fuses on input as the cheapest solution for overcurrent. Also I'd recommend to use switching power supply as LDO converting from 12V to 3.3V would have to dissipate A LOT of heat, especially under load. If you don't need more than ~1A of current - use TPS6300X or TPS6120X (I prefer fixed voltage ones - less components), if you need a lot of current - use LM3152MH with appropriate MOSFETs - you can basically choose any current you want. Also use a normal automotive fuse before your board - just in case.