E.P.
Hi. I remember the feelings i had before our second baby came home... so much worry about the older one and how to transition her... I didn't want everything to change yet the gift of a sibling is truly worth it all. My daughter was not quite three when we added our baby and overall it wasn't too bad --but there is definitely an adjustment. There are some cute books that help to create that excitement of being the big sibling. Just search thru amazon and you'll see a few on that topic.
Once the baby comes, it does help to have a gift for the big brother and always refer to the baby as "our baby". Just including them in everything makes them feel a part of the family. You will definitely need to carve out one on one time with the big brother and as someone already mentioned, the baby is not aware that they are getting shorted! I would say that you have to work hard to never show the true exhaustion or frustration when you have those moments... just because the 3 yr old is watching you and taking your cues of how to react to the baby.
Someone once told me it is really a 6 month period of acceptance and by then, they can't even remember what life was like without the baby.
Lastly, i think its important to acknowledge your son's feelings of jealousy and empathize that it is hard to get use to someone else and especially when mommy is so needed by the baby. We thought three was a great age to become a big sibling since three year olds start getting into friends and other activities more independently.
anyways, congrats and best of luck!