The triple test or triple screen is a universal test that is done in all women when they are pregnant, particularly around week 12 of pregnancy. It pays special attention to the risk of Down's syndrome in the fetus by combining two markers from the first trimester ultrasound with two hormones examined in pregnant women (BHCG + AFP) along with her age.
In case your OB/GYN refers you directly to invasive tests (amnio test or chorionic villus sampling) because you've been pregnant before and chromosomal abnormalities were detected in the fetus, then a triple test would be unnecessary.