Hmm... I don't, at least. I like to keep some units in reserve so that they can charge into the melee at a critical moment, stamina fresh. But the three-line formation is another thing altogether. I don't really know, but I think the people who use the three-line formation use it for maximum historical accuracy.

If I have all three classes of Roman infantry on the field, I would put the triarii at the edges, to anticipate cavalry charges at the flank, and so that as the triarii beats back the enemy troops, they can wheel around to outflank the enemy line. The hastati and principes I'd put in an alternating pattern: hastati, principes, hastati, principes and so forth.