Transform > Shear works well if you have a group or simply a bunch of objects selected at the same time:
- few rectangles, grouped
- scaled horizontally to 75% and vertically to 86,6025% to keep the apparent size (
- Object > Transform > Shear > -30 degrees is applied
- Object > Transform > Rotate > -60 degrees is applied
I guess you want automatically vertical parts along, too if you are making something 3D-like. For it there's 3D effect Extrude & Bevel:
In the left the effect is applied to the group. Every part get the same extrusion depth.
In the right the parts are ungrouped and everyone got different extrusion depth. Unfortunately they also lost their places, they are dragged manually to the same formation as the extruded group.
Applying Object > Expand Appearance and Ungroup frees the exrtuded objects so that every polygon can be edited separately. You need 2 Ungoupings if the extruded item was a group. You need it if you for example want to change colors or insert strokes. Effect Extrude & Bevel generates very complex results if the extruded shapes have strokes. That's why I had no strokes.
Expanded separately extruded shapes can be placed to the isometric formation easily if you have the same shapes extruded as group + expanded as your reference and if you have snap to point and smart guides ON. Select one shape and drag its corner with the direct selection tool.
ADD due comments:
I guess you reach something unusable. Isometrization as an effect which can be disabled really could allow easy edits to the original, but you will lose Snap to Point. Snapping will use the non-effected version.
Of course you could have an expanded version of the isometrized result to have snaps and hide it when it's not needed.
Preferably have a replica of the non-isometrized original in a hidden layer, use the replica if the original needs edits. Delete the isometrized version, make the edits, make a new replica and isometrize. Have some control points to be able to move the new isometrized version to its place with snapping. It moves if its centerpoint is changed due the edits.
The straightly knocked version 1...4 can be reversed. You can rotate, shear and scale to opposite directions. Scale needs better value for cos(30 degr) than 0,886 for full Illustrator's accuracy. Use 0,886025. Its reverse is 115,4701%