Please help with the sentences below.
Buffett noted that Greenspan said in 2002 that the current account deficit—then $350 billion—must be restrained. It has doubled since then. The U.S. is a net debtor to the world to the tune of $3 trillion.

Q1: There's no context in fact, and what is the $350 billion? I've referred to the internet and found that President Bush was trying to reduce the tax by $350 billion in 2002. So I wonder whether the $350 billion in the passage is the one President Bush trying to reduce?
Q2: It has doubled since then. What does 'it' here refer?
Q3: Does the last sentence mean that The U.S. has be in debt of $3 trillion?

Question

Please help with the sentences below.
Buffett noted that Greenspan said in 2002 that the current account deficit—then $350 billion—must be restrained. It has doubled since then. The U.S. is a net debtor to the world to the tune of $3 trillion.

Q1: There's no context in fact, and what is the $350 billion? I've referred to the internet and found that President Bush was trying to reduce the tax by $350 billion in 2002. So I wonder whether the $350 billion in the passage is the one President Bush trying to reduce?
Q2: It has doubled since then. What does 'it' here refer?
Q3: Does the last sentence mean that The U.S. has be in debt of $3 trillion?

Answer

The $350 billion from question 1, and the 'it' from question 2, both describe the 'current account deficit'. The word 'deficit' means the difference between how much money is coming in, and how much is going out.

So in 2002, the government spent $350 billion more than it earned. It (the deficit) has doubled since then, so in a given recent year the government spent $700 billion more than it earned. And yes, for Q.3, 'the U.S. is a net (net = total) debtor ...', means the U.S. owes the world approximately $3 trillion in total.

Essentially, deficit is the amount of new debt created in a given year. The net debt is the total of all the debt built up over past years.