Most were retained by the Federal govt and from there they disapear quickly. Many were sold abroad, melted down & reforged into something more modern etc. Only a relatively few were retained for monuments.
The last reported use of the 3" Rifle was by the Japanese against the US in the Phillipines... man of those fine 3" guns were converted into breach loaders and soldiered on well after the ACW in various armies across the world.
THe French became so enamored w/ the 3" Rifle that it would become the influence for the famous "French 75" and 75mm was a pretty standard caliber world wide until well after WW2.
This book has some fine details on CW
arty:
Hazlett, James C., Olmstead, Edwin, Parks, M. Hume,
Field Artillery Weapons of the Civil War, University of Illinois Press, 2004.