Get bounds/center/extent/etc of multiple NodePaths

I understand the way to get the center of a single NodePath:

np = NodePath('test')

What I’d like to do now is find the combined center of multiple NodePaths, e.g.

np1 = NodePath('test1')
np2 = NodePath('test2')
np3 = NodePath('test3')
# some code to return center of np1, np2, and np3

Is there a built-in way to do this, or will I need to do the arithmetic myself? Thank you!

It is easiest to reparent them all to a single parent node and querying the bounds of that.