the problem may also be that your front is only 4 units wide. What I mean by that is that the game puts an artificial limit on the number of units you can control, so in order to field the 3 ranks, you use a 4-unit front. That's fine, so long as the terrain covers your flanks (ravine) or your enemy only has a few units. If the nme has a front line 10 units long, the Romans would have deployed 10 of each unit maybe.

Given how the barbarian AI lines up, even anciet Roman commanders would have sacrificed the 3-lines in order to avoid being outflanked. I think the trick is to fight like anciet Romans, but when deciding your formaiton...

FIRST: ensure you have the same frontage of the nme. Only after the frontage is about even should you worry about maintaining that front in the traditional 3-rank style.

One last thing, try not to have forest on your flanks. While it may have been preferable in reality, in RTW it doesn't mask cavalry, and it also strengthens the barbarians, which is doubly-bad