In order for a human to consume enough marijuana to be fatal, they would have to consume nearly 40,000 times the amount of THC required to intoxicate them. In contrast, it only requires about 5 to 10 times the amount of alcohol required to intoxicate, to be fatal.
If it requires 3 beers to intoxicate you, it only requires 15 to 30 beers to kill you. However, if it takes you 3 'hits' of marijuana to intoxicate you, it would require 120,000 hits to kill you. Thus, it is virtually impossible to die of a marijuana overdose.