If I have capital gains that are excluded from federal AGI, will that income be exempted from State AGI calculations and therefore State income tax as well? I'm specifically interested in:

1) NY
2) NJ
3) CO

Thanks for any expertise you can offer!

It depedns; soem state ,i.e., OR state yes ;Depending on the state, the two can differ by quite a lot as some states don't follow the federal tax code at all. And some states don't have a State AGI. So it can be possible for the state AGI to be a different number than the federal AGI is. You may visit your website and get a copy of the blank states income tax returns and look at them and see if you can determine if their would be any difference in the numbers.
it could also be possible that you would not have any change
Or maybe you could contact your states tax departments for accurate info.