good morning house, trust you all slept well?

thou this my question may look odd but it has been bothering me of recent.

we all know that two children born of thesame woman in thesame day and from a single pregnancy are called twins, while three children born of thesame woman in thesame day and from a single pregnancy are known as triplet
my question now is... if a second twin is refer to as a twin brother/sister how do we call him or her if they happen to be triplet?