Consider the situation where I have two spaces screens. From the second screen I open a terminal and make it fullscreen.
By doing so I expected cmd+3 to take me to the terminal app, but it doesn't work.
Any ideas on how to make it work?
You can simply do ⌃ → (default) to move one space to the right, which is where your app should be. You can change these settings in the Keyboard preferences:
An alternate is to use Cmd-Tab to bring up the applications list and navigate.