Kannagi (கண்ணகி) is the heroine of the Tamil epic Silappatikaram ("The Tale of an Anklet," c. 2nd-3rd c. When her husband Kovalan was wrongly executed by the Pandyan king, she came to the court and burned the city of Madurai to the ground with the fire of her chastity.
Kannagi 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 Kannagi reduce to 3, The Communicator. This is a traditional interpretive system, not a factual claim about the name.