Got a used 3" black diamond lift w/ a add a leaf on stock springs. have a extra set of stock leaf springs. the question is should i take one of the leafs out of the extra set to help prevent sag?
To "prevent" sag? Probably not. Adding any leaf to a spring pack will raise the vehicle. For example, on a completely stock 1988 Laredo, taking another spring pack and cutting the eyes off the main leaves to use as full-length AALs netted about 1-1/4" of lift.
If your Jeep is at the height you want now, leave it alone unless/until it sags, then start adding leaves. OTOH, if the back is not as high as you want or need, by all means use the leaves from the spare spring pack to even things out.