This website gives you a chance to spend that much money.
I assume I were a US citizen and tried to do the following:
$2.5T Make poverty History
$952.1B (universal health care for every American - 300M people)
$420B to overhaul of the nation’s energy infrastructure to switch it to solar
$100B (%5B per year for 20 years) to teach every man, woman and child on the planet to read
$10B Increase sustainable Organic Produce in the US
$3M for Non-Violent Leadership Training for 10 Million Leaders (10 years)
$3M [The last item is too cheap! Add a similar amount to donate to UN for them to spend]
Well it is too hard to spend 3 Tillion dollars.
What will you do?