Dynamically set zoom level based on a bounding box

You could simply use:

var group = new L.featureGroup([marker1, marker2, marker3]);

map.fitBounds(group.getBounds());

Using @Farhat's answer, I figured out that all I needed was to pass an array of arrays:

map.fitBounds([
  [-4.8587000, 39.8772333],
  [-6.4917667, 39.0945000]
])