Event Receiver - local site for debugging

I am working on few sub sites (all have same structure and lists) and need a "list item receiver". Here is the site structure

Site Collection / OSR / site1

                                 / site2

                                 / site3

I created the event receiver in visual studio.

(1) I deployed the ER as farm solution with "What local site do you want to use for debugging" = Site Collection. The event receiver does not fire.

(2) Then I deployed the ER as farm solution with  "What local site do you want to use for debugging" = site1. The event receiver works.

My question is do I need to deploy the ER to site1, site2 and site3 separately? Is it possible to have one ER take care all sub sites?

Thanks for your advise.