You can only use one image at a time to execute commands. All of your steps execute in the first container that is listed. Any secondary containers are there as “services” – so they are designed to run things like redis, MySQL, etc.
If you need an image with both node and python3, I don’t think one exists at the moment. You can either make your own image, or install python3 as a part of the build. (or vice versa, install node as a part of the python container).