I don't really use Rome, but a fellow Forumer wanted to see the 3-infantry line Roman Army at work. This is what I came up with (for the 10k denarii level).
Just before this battle, I slaughtered the same opponent's Gallic army. But we all know barbarians GENERALLY get slaughtered online so we opted for a Rome vs Rome battle for the next game. I'm the Barbarian_General during the game (usually known as the Prince_of_Macedon).
Anyway, the biggest fear of using 3 lines is that the center will be surrounded easily. In this battle, the enemy threatens me with an enveloping attack with his superior cavalry force. But by keeping my formation compact, I'm able to negate his cavalry superiority.