I doubt it'll be a 3/1 ratio, but that's a valid point. Hopefully, most of the jobs lost will be those where companies simply don't replace an older driver after they retire. While automation will probably hit the linehaul side of LTL harder than other areas, OTR drivers will still be needed for irregular routes, and local/regional will probably gain jobs.
I think the cost of the technology, especially as it relates to ongoing maintenance, will create a barrier to how far automation can practically spread. Simply put, I don't think there will be enough people capable of fixing these trucks, the ones that are won't be spread out evenly around the country, and they'll be insanely expensive to hire.