There is an error in the first calculation.
3 billion
sestertii at 20 grams would equal 60,000 MT.
Though of course as Alex notes the inventory would likely have been silver and/or gold.
Interestingly, we know that in 1058 AD the Song Dynasty mints in
China produced 8,000,000 strings of cash coins. As each string was 1,000 coins this translates to 8,000,000,000 coins which weighed around 23,273 MT.
SC