I would also opt for numbers instead, but for different reasons.
Gaius makes a good point about inferior troops routing prematurely, especially if both armies have firing drills, so a crossfire might not be the most surefire way to defeat smaller, but higher-quality units.
My approach has always been to disrupt the enemy firing line while preserving my own, so in a case of two inferior linemen units against one very experienced one, I flank the enemy unit with one while the other engages in a one-on-one volley exchange.
Flanking unit then charges, disrupting their line, while my other unit continues firing. Friendly fire is a factor, yes, but with superior numbers you can afford to lose men anyway. And if you flank correctly, you take out much more enemies than friendlies.
You don't even have to engage in melee all the time- just charge, withdraw, and charge again, if your micro skills are good enough, and you have the luxury of intensively focusing on a small part of the battle. You just have to do enough so that his more-powerful firing line never gets a chance to reform and fire back.
Of course, it'll depend on the quality and stats of the unit. I might not, for example, charge Austrian infantry against veteran Prussian troops,
because of this.
Veni, Vidi, Pwni.
Read my after action reports for my Austrian campaign:
Broken Alliance