Can I copy all the apps from one Roku Express to another?
Normally, all Rokus connected to the same account get the same apps automatically. I suppose there might be some rare exceptions like apps no longer in the channel store or a content provider that is having a dispute with Roku.
.. or adding in an older Roku that is not supported by some of the newer apps.
Can you tell me how to copy the apps from one to another
If you link it to the same account, then they should start downloading automatically.
How do you know, if all three device are link to the same account
You can go to settings -> system -> about on each Roku.
If some of these Rokus are not yours, then I would not put them on the same account and would instead, just download apps manually.