Why 3-Stars

Often one might see three-stars in an enemy rally.

So the question is, why would people use these three-stars?

Well, for starters, the lower army power might cause you to face weaker enemies.

And with weaker enemies, you potentially have a higher chance of winning.

Firstly, three-star arbalists are actually pretty decent tanks.
Then, three-star archers work REALLY well with Edward IV (powerful procs + good proc rate = DECIMATION!)
Three-star musketeers can deal approximately 80% of the AoE damage Sarky does, being miniature Sarky's essentially.
Polymorphing is really annoying in large amounts (renders armies inactive and unable to do anything), leading to quick enemy destruction (especially when teamed up with musketeers).
Healing with the three-stars is decent, but only with heroes that don't rely upon their minions.