The problem is that you're guarding in both cases with the same hashrate or miner cost to be more precise an ecosystem worth 500 billion and one worth 10 trillion. So if 3 billion might sound like a lot to attack a coin with a market cap of 500 billion it doesn't sound that excessive when taking down one that rivals the GDP of the USA.
What kind of attack can one perform to own 10 trillion worth of BTC? The only feasible attack would be double-spend, to perform a 3 billion $ attack you would really want to reverse a transaction/ series of transactions that is worth much more, a double spend attack is not feasible now or with a market cap of 10 trillion as long as:
1- The attack is very expensive.
2- The profit of playing fair is high.
A government carrying out different attacks just to damage bitcoin is something different, you could argue that if needs be, it won't matter much how much hashrate is securing the network, if they label mining bitcoin as a crime worldwide, 90% of miners will shutdown and then it will cost them little to nothing to attack it.