How can I add a thick outline aroud the group formed by the three vector rectangles ?

I managed to do something "by hand". But it won't follow any change in the starting shapes.

I would duplicate the objects (ctrl D) and join the duplicates (ctrl +).

Then you can set an outline colour on the new object and set the fill colour to transparent.

Assuming that you want to keep the original objects.. if you don't, skip the duplicate part. ;)

Having an outline change on-the-fly... I don't think there is currently anything for that.

Feature requests can be made: https://inkscape.org/forums/beyond/how-to-report-bugs-or-request-new-features/

Is (ctrl+) the same as path-union ?

Yes.

Yes, it is. I mistyped.

If this is for animation, maybe the outline filter would work? It won't give you an outline path but rather a bitmap outline.. I think.

Filters -> Morphology -> Outline

Here's another method using clone and boolean path effects. Use the Node tool to adjust the rectangles and see what happens to the dashed outline. 

Don't use the Select tool to adjust the shapes. It applies shape transforms and distorts the outline.

My ancient laptop stutters a little recalculating the path effects. More modern machines probably won't notice. 

Grouping shapes or moving them to another layer can mess up the links and the geometry. I need to investigate before I report a bug.

Thant's interesting. Do you have to use any special options in the path effects?

bleke

Do you have to use any special options in the path effects?

Yes. It's not completely straightforward.

Draw three rectangles. Select all and create clones [Edit > Clone > Create Clone] or [alt+d] to creates SVG clone objects. These are not the same LPE clones which are path objects duplicating the original nodes and paths.

Open the Path Effect dialog [shift+ctrl+7]. Select a clone and click the [+] button to convert it to an unstyled LPE clone. Unset the option [Allow Transforms] so that the clone keeps the same position as the linked rectangle. Repeat for the other two clones.

Choose one of your clones as the "master" and set the fill and line styles. In the LPE dialog [+] [Boolean Operation] [Operation: Union]. Select another clone on the canvas and copy [ctrl+c]. Select the "master' again and in the LPE dialog [Link to item] to create a union shape with the clipboard clone. To add another shape to the union, add another boolean LPE to the "master" and link to another clipboard clone.

It works!

I would never have figured that out in a million years.. :)