Which 3 years?1862 to 1864. The CPI increased 14.2% in 1862, 24.8% in 1863 and 25.1% in 1864. In total the CPI went from 8.54 in 1861 to 15.23 in 1864 an inflation of 78%.
Source = Measuringworth.com
We went off the gold standard with the First Legal Tender Act in 1862. We didn't get back to it until after the war. All that inflation from 1862-64 was caused by the mass issue of what became known as the ‘greenback’ (which was fiat currency issued without backing)in order to finance a little dust-up mistakenly called the ‘Civil War’.