When a mammal gives birth to three offspring at the same time, each is a triplet. If you're a triplet, you've got at least two siblings right from the moment you're born.
While some triplets are identical (three fetuses from a single egg), it's more likely for triplets to be fraternal (from three separate eggs). In humans, twins are more common than triplets, and multiple births of more than three are even more rare. Triplet can also mean another group of three, like three lines of poetry or three notes of music.