See the section "dynamically creating objects" in the manual section on the C3 runtime.
All you need to do is place all objects the layout will use on the layout, and destroy them on startup if you don't need them right away. Then Construct's memory management will handle everything smoothly. This was true in C2 and is still true in C3, except the details about how dynamically loading objects that were not already pre-loaded are different (since it's now done in a way to improve performance and reduce in-game jank).