While Tessa is now preferred, this slightly sharper diminutive of Theresa was the more familiar option in the early 20th-century. It enjoyed a brief return to the US charts in the 60s and 70s but is currently given only to a handful of girls each year.

Amphitrite is a mysterious Greek mythological figure, most often cited as a sea goddess who was the wife of Poseidon. She was said to be the personification of the sea water itself and was said to be present at the birth of Apollo.

Never as well known as that other Roman numeral name, Octavius, Tertius just might hold some appeal for the parent seeking a really obscure name with the patina of antiquity for her third son.

An interesting international possibility for a third son.

Pretty, zippy and almost unknown, this Italian name would suit a third child.

Tertia—which is occasionally used in the UK, but rarely in the US, would make an unconventional but interesting possibility for the third child in a family. Terza and Terzo are lively Italian female and male variations.

Strong and vital Asian birth order name.

It's difficult to conceptualize the numerical value of a trillion, but this extraordinarily large number name is a rising choice for baby boys.