I am not even talking about 3 Million, let's say he used the average price between 2016-2020 was $10000, this would still translate to $590000,
It doesn't work like that, the average was well below 10k for that time and poeple who buy
BTC don't buy the same amount no matter the price, you can check LB volume for example, and see how in 2016 was 10k BTC a week and after the spike in price went down to 1k although in $ terms it went up.
So probably most of those coins were bought in 2016 and far less in 2020, with the same ratio as the price.
Certainly you should sue the scammer. However, people will definitely ask how you didn't notice that someone stole your BTC. and they stole it very slowly. 1721 transactions? This doesn't look like what a scammer woudl do.
It can be possible, a vulnerability in the code or a backdoor installed by the manufacturer or somebody that had access to either the machine or the wallet so it constantly grabbed deposited coins, not impossible that the thief himself came and sold coins for $ knowing he could get his coins back from the compromised machine. As for the time and length of the "hack", MtGox went for months with half a million coins missing from their wallets.
That aside, the story is weird not because of the theft but because of how things went.
So he wants to sell the stores, somebody comes with an offer, asks for the hot wallet? Neah! How has this buyer already identified who's done the transactions, why would a buyer buy the shops and the coins, OP knows he has at least 59 coins but never bothered to check his balance although already for a year he would have been a multi-millionaire by the amount he thought he had in coins alone?
Too many things that don't make sense.