Use Selection Center

Main toolbar > Use Selection Center (on Use Center flyout)

The Use Selection Center button, available from the Use Center flyout, lets you enable rotation or scaling of one or more objects around their collective geometric center. If you transform multiple objects, the software calculates the average geometric center of all the objects and uses that for the transform center.

The axis tripod shows the center that is currently being used.

Note that transformation center mode is set on a transform-by-transform basis, so select the transform before you select the center mode. If you do not want the constraints to change, turn on Customize menu > Preferences > General tab > Reference Coordinate System group > Constant.

With the Selection Center option, an averaged coordinate system is used to rotate the objects.