Concerning the animal offered for the 'aqiqa [sacrifice offered on the birth of a child] of three girls, which is preferable in Islamic law—offering three sheep or one calf equivalent in value to the three sheep but which will provide more meat?

In this case, it is preferable to slaughter what is of greater benefit and provides more meat. As long as the calf fulfills the specifications of a sacrificial animal, it is better than offering three sheep since it fulfills the sunnah for seven people.
Allah the Almighty knows best.