A question regarding lyrics of the well known Irish “anthem” called “A Nation Once Again”. There is a line I do not understand the reference to.

"When boyhood's fire was in my blood
I read of ancient freemen,
For Greece and Rome who bravely stood,
Three hundred men and three men"

Who does these 303 men refer to?
The song was written by Thomas Osborne Davis (1814–1845). Who was a founder of Young Ireland.
Many thanks,
Len Wallace, Windsor, ON