I think you may be thinking of the LD50 for rats. People have died from much less than 3g/kg.
The lethal dose was estimated to be less than 10 g of sodium (<5 teaspoons of salt) in two children, and less than 25 g sodium in four adults (<4 tablespoons of salt).
https://pmc.ncbi.nlm.nih.gov/articles/PMC5537768/
An acute toxicity from excess sodium intake with the possibility of fatal outcome has been reported in relation to the ingestion of huge amounts of sodium, such as 0.5–1 g of salt/kg body weight.