i. Where do we put the business logic in a Spring application? This is no different then any other application you may write. Typically the business logic lives in the service layer, how you want to organize your classes and packages in the service layer is entirely up to you, there are a couple different
patterns for package structure out there and googling around will demonstrate them.
ii. Can we use EJBs in a Spring application? Yes.
iii. Is it good to use EJBs in a Spring application or does Spring provide any alternative to this? I would argue just because you can does not mean
you should. I have never encountered a need to use EJB's in my Spring applications and I have worked on some pretty large applications. To the second point, Spring was right from its inception considered an alternative to EJB's, so I guess the answer is yes.