I'm 28 & had a leep to remove CIN 3 from my cervix. I'd like to plan for a 2nd child soon. How do you think a pregnancy could be affetced by a leep?

Shortened cervix. It really depends on the size of the cone of tissue removed from the cervix. The critical measure is the cervical length measured on ultrasound. If it is 2.5cm or longer, unlikely to have any effect on cervical competence. Less than that i would recommend a mcdonald cerclage (stitch). The cervix may have scar tissue that could slow dilatation in labor.
Should be fine. After a leep for cin iii, your doc can tell you if all the abnormal cells were removed. They'll also make sure a cancer wasn't hidden in your cervix and ensure neg surgical margins. If your next pap is normal, it's usually ok to conceive. Cervical stenosis (blockage) is rarely a problem & a theoretical risk of incompetent cervix is rare. Your OB can follow your cervical length once you conceive.