Amazon's market cap is 1.6 trillion to put the 3 trillion figure in context.
The financial resources invested in these wars was enormous.
Plus acquisitions and operations/maintenance eat up much more than personnel costs anyway, and the surplus of that money goes to defense investors (who I personally don't enjoy subsidizing so much).
Instead of killing people, and destroying their civilization's infrastructure, we could have instead spent that on R&D for our own energy independence. More of that money would have stayed in the country, enriching us all and boosting our own economy rather than producing goods used for destruction.
Even not doing R&D, but just putting that money into basic infrastructure in the USA, like roads and bridges, would promote local economic growth.