gistfile1.cs

var save = new ButtonExtras(ButtonType.Custom, "Save", "Saves this modified post to your blog");
            var saveAs = new ButtonExtras(ButtonType.Custom, "Save As", "Saves this blog post as a local markdown file");
            var publish = new ButtonExtras(ButtonType.Custom, "Publish As", "Publishes this post to another blog, or as another post");
            dialogService.ShowConfirmationWithCancel("Markpad", "What do you want to do?", "", save, saveAs, publish);

            if (save.WasClicked)
                return base.Publish();
            if (saveAs.WasClicked)
                return SaveAs();
            if (publish.WasClicked)
                return DocumentFactory.PublishDocument(this);

            return TaskEx.FromResult<IMarkpadDocument>(this);
        }

        var save = new ButtonExtras(ButtonType.OK, "Save", "Saves this modified post to your blog");
        var saveAs = new ButtonExtras(ButtonType.Yes, "Save As", "Saves this blog post as a local markdown file");
        var publish = new ButtonExtras(ButtonType.Retry, "Publish As", "Publishes this post to another blog, or as another post");
        var result = dialogService.ShowConfirmationWithCancel("Markpad", "What do you want to do?", "", save, saveAs, publish);

        switch (result)
        {
            case DialogMessageResult.OK:
                return base.Publish();
            case DialogMessageResult.Yes:
                return SaveAs();
            case DialogMessageResult.Retry:
                return DocumentFactory.PublishDocument(this);
        }

        return TaskEx.FromResult<IMarkpadDocument>(this);
    }

Currently the dialogservice call doesn't return the dialog result. It consumes it and changes it to a bool. This scenario was thought of, but not fully implemented.

Also the dialog service was a bit of an experiment on my part and you're suffering the consequences of a poorly thought out API surface. Sorry.