Procedure to apply service packs on replicated enviroment

Hi All,

I have a question on how to apply service packs on a replicated environment.

I have transcational replication configured on 2008. All are Enterprise Editions.
Now, i want to apply the service pack upgrade, what should be the order of applying the service pack and what is the reason behind if one is choosen first over the other.
Also, do I need to disable all the sql agent jobs or stop sql server agent so that replication activity can be performed on the servers?
Are there any  pre-requistes to be taken care bfore apply the patch?

Server-A:- Publication
Server-B:- Distribution
Server-C:- Subscriber

Thank you.