Use Selection Center
Command entry: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, 3ds Max 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.

NoteThe 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 center setting 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.