If the estimate of my baby's gestational age is off, can that affect my Triple Screen test results, and can an abnormal result be an indicatation of twins?
You bring up two considerations with your question. The triple screen test is helpful in screening for neural tube defects (such as spina bifida) and Down's syndrome, and it is currently being investigated as a possible red flag for pregnancy-induced hypertension. (PIH), previously called pre-eclampsia, and before that, toxemia. But because the concentration of maternal alpha-fetoprotein is related to gestational age, it can be falsely alarming or falsely reassuring if the dating of the pregnancy is incorrect.
And yes, it can be incorrect for multiple gestation, as in twins.
Either way, if it falls out of the normal range an ultrasound is indicated, preferably a "level three" ultrasound performed by a maternal-fetal medicine specialist (perinatologist) to see if there are any structural presentations on the baby that would warrant an amniocentesis. This would also lay to rest any suspicions of twins.
The call is up to your doctor to tell you whether the actual numbers you report are out of range—they may vary per type of population.