Not all have risks. Sometimes preterm labor, and even delivery, can happen in women with no known risk factors. If a pregnant woman doesn't have any of the typical risk factors like twins or triplets, bleeding, prior history of preterm birth, etc, she should call her OB if signs or symptoms of premature labor. If she does have those risks, she should be prepared to notify her OB and go to labor & delivery immediately.
Many. There are many risk factors for preterm birth. The best predictor is a previous preterm delivery with a prior pregnancy. Other risk factors include multiple gestation, short cervix, cigarette smoking, being a teenager, short interval between pregnancies, drug use, poor nutrition and being underweight or very overweight.
Triplets. The biggest risk factor for preterm delivery with triplets is - having triplets. I'm not a maternal-fetal specialist, but as an rei I've seen this alot - the main risk factor is the triplet; age, weight, etc, contribute a very small risk above and beyond.
None/triplets suffic. No other factors are required; triplets in and of themselves will increase the risk of preterm birth tremendously. In fact the mean gestational age at triplet birth is 32 weeks and ~100% of triplets deliver prematurely. As you can tell all other risk factors pale in comparison. There is no evidence that cervical cerclage or Progesterone can prevent preterm birth in twins or higher multiples.