Is it normal to get darker skin in butt crack in 3rd trimester of pregnancy?

Yes. Many areas of the skin darken in pregnancy nipples, pubic area face etc. These usually lighten after delivery.If you are concerned show ob, for possible referral to dermatologist.
Yes with pigmentatio. I am not sure about the butt crack.It does not happen to all. Pigmentaion may be effected due to hormones. It may also be all over one's coloration. Nothing to be afraid of.It should gradually get bak to normal color. Just focus on your beautiful baby!