How to make 3 different sized canvases, which lets you work on one and affects the others at the same time.

You can prepare 3 different canvas (as you wish) and then prepare largest design (some layers, masks, texts, vectors) as a single Smart Object placed to all of your canvas. You can also use external PSD file linked as Smart Object to all of your canvas.

Then open all canvas and smart object (use split view to see all documents) and make changes to smart object.If you save this all of your canvases will update

pawel

All your canvas have the same aspect ratio square. You want all size canvases to have the same content.  So all you need do it to create the large canvas documents.  Then use a plugins script like Image Processor Pro to batch  create all the smaller canvas size versions.

First, create multiple artboards of the desired size within a single document.

Then, use Smart Objects as macpawel suggested above to replicate the content to each artboard, scaling as needed.

If you want to have multiple views of the same file in Photoshop (for example, you are working a close-up detail on one monitor, and want to have the full image as a reference on a second monitor, or in this case, viewing one artboard and seeing the changes on a different artboard):

Window > Arrange > New Window for [file name]

You can create multiple windows for a single file, and can use Window > Arrange to tile or cascade them, or just drag the tabs to the desired locations.

Mike

Seems like doing it the hard way to me.  If all have the same aspect ratio and content.