I am managing multiple EC sites at EC - CUBE 3. Can I install Payder plug-in on multiple EC sites?

Yes. Can introduce.
However, it is necessary to register multiple shops in Payder and obtain the API key of each shop.
3 store EC site operation ⇒ 3 shops Payder shop registration ⇒ 3 store Please set a different API key.
* 3 If you use the same API key at a store, it will not work properly. Be sure to register each shop in Payder and use different API keys for each shop.