So I'm I'm firing with my ABC and Fearlessness adds a [hit] does that mean I have 4 unblockable hits? Same for Finn adding a blank re-rolled into a hit with ABC.

Fearlessness will add its hit to the unblockable hits from an Autoblaster Cannon. You still need to get hit results on those three dice. Kavil's ability interacts with Autoblaster Turret in the same way.

Finn doesn't work with any attacks but primary attacks so you are unable to use him with an Autoblaster Cannon.

Thanks

if your comparing which is better for list building reasons Fearlessness is scum only