Tertia is from the Latin tertius (third). Aemilia Tertia (c. 230-162 BCE) was the noble Roman matron and wife of Scipio Africanus, the conqueror of Hannibal — mother of Cornelia Africana, who in turn was mother of the Gracchi brothers.
Tertia does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Tertia reduce to 1, The Leader. This is a traditional interpretive system, not a factual claim about the name.