Unfortunately Construct 3 doesn't offer any great tools for isometric games.
Although it is possible to do, the editor isn't doing you any favor.
However there are official isometric pixel art examples you can look at (which use tiles i believe) so lookout for them.
Isometric examples can also be found in the asset store.
You can use sprites and place them manually, which is rather time consuming.
You can program your own level designer using C3.
Or you can use external tools as a map editor, export the map to json. Then import the json format to C3 to generate the map on runtime.